Drivetrain

Rear axle, differential & driveshaft

The core pick stands: a Currie 9-inch bolt-in housing and 31-spline axle package, 5-lug (GMC7387X5, $1,550-1,750), with a TwinTrac or Trac Lock third member in a Ford ratio, rear discs, and a custom 1350 driveshaft. The Ford ratios are 3.50 for the TH400, 3.70-3.89 for the 4L80E and 3.89-4.11 for the T56, replacing…

Prices checked 2026-10-12 via web-search result snippets and vendor pages that would load. Summit/JEGS pages block fetches, so those prices come from search snippets and are marked "snippet". Retail prices move; treat every figure as plus or minus 10 percent. Anything I could not confirm is labelled "not confirmed".

The pick

Good
Rebuild the truck 12-bolt: about $1,400-2,600. That is gears $225-410, overhaul kit $319, carrier $520-920 (Auburn $521, Eaton Posi $680-751, Truetrac 913A315 about $725-764, Dura Grip $920), and pressed-bearing 30-spline axles $335-700.
Better
Currie 9-inch complete crate with TwinTrac third member and 31-spline axles, CE-GMC7387X5-07, $3,150. A la carte it is housing and axles $1,550-1,750 plus third member $1,400-1,750, then rear discs from $771 (CPP DBR320-V) and a driveshaft $625-1,000. Total is roughly $4,500-5,700.
Best
Currie or Strange 9-inch with Wilwood Superlite rear discs ($2,142) and an aluminum 1350 driveshaft ($625-1,000+). Or a Detroit Speed QUADRALink housing ($2,048, needs the 4-link) or a Strange Dana 60 ($2,525-2,895). Roughly $5,800-7,500+.

Pick: Currie 9" housing package (GMC7387X5, 5-lug, 31-spline; $1,550-$1,750) with a 31-spline TwinTrac or Trac Lock third member in a Ford ratio (3.50 for TH400, 3.70-3.89 for 4L80E, 3.70 for T56 with the 0.63 sixth, 3.89 as the alternative), plus Explorer or POL rear discs, and a custom 1350 steel or aluminum driveshaft with a 32-spline yoke for TH400/4L80E. Budget about $1,550-$1,750 for the housing/axles and $1,300-$1,750 for the third member (or $3,150 for the Currie complete crate CE-GMC7387X5-07), $500-$800 for rear discs, $600-$1,000 for the driveshaft. Reasons: it retains stock width, perches and shock mounts; all three transmissions fit it; a wide ratio range is available (Currie lists 3.00-4.11) [32]; third-member swaps take an hour, which suits a decade-long build; and it has no C-clip axles.

If cost-constrained or on Path B: rebuild the 12-bolt (after confirming it is a truck 12-bolt) with Yukon or Richmond gears, an Eaton Truetrac (913A315) or Auburn carrier, the Yukon overhaul kit and a set of pressed-bearing 30-spline axles ($1,400-$2,600 total). A fair counter-view from gmsquarebody posters is that a stock 12-bolt holds 400-500 hp and traction is what breaks parts [4], so a rebuilt 12-bolt with a 3.42-3.73 gear is a legitimate long-term choice for a 400-450 hp Path B truck, not merely a fallback. Do not combine a 12-bolt with 500+ hp and wide sticky tires.

Gear ratio by transmission. Important correction: the ratios in the original draft (3.23, 3.42, 3.73) are GM-axle ratios. A Ford 9" does not come in 3.23/3.42/3.73. Currie's catalog ratios are 3.00, 3.50, 3.70, 3.89 and 4.11 [32]. If you stay on the GM 12-bolt, the GM ratios apply; if you go 9", use the Ford ratios in the right-hand block. RPM at 70 mph, 28" tire at spec diameter (my calculation; 4L80E overdrive 0.75; T56 Magnum 6th = 0.63, the only verified shipping sixth - the 0.50 sixth assumed in the first draft was discontinued by Tremec in 2017, see section 08). Tire-diameter basis (added in the integration pass, 2026-10-12): the rpm figures here use spec-sheet (unloaded) tire diameters. Published revolutions per mile imply a loaded rolling diameter about 3% smaller (Nitto NT555 G2 275/50R17: 27.87 in spec, 746 rev/mi = 27.03 in; BFGoodrich Radial T/A 275/60R15: 28.0 in spec, 743 rev/mi = 27.14 in; section 91), so real cruise rpm is about 3% higher than tabulated. Example: the recommended 4L80E + Ford 9" 3.70 + 275/50R17 is 2,175 / 2,343 rpm at 65 / 70 mph at spec diameter and about 2,240 / 2,415 measured. The recommended rear tire is 27.9 in spec (27.0 in rolling), not the 29-30 in tire some tables here assume; choose the gear after the tire.

Axle ratio Axle TH400 (1:1) 4L80E OD T56 6th (0.63)
3.00 Ford 9" 2,520 1,890 1,588
3.23 GM 12-bolt 2,714 2,035 1,710
3.42 GM 12-bolt 2,874 2,155 1,811
3.50 Ford 9" 2,941 2,206 1,853
3.70 Ford 9" 3,109 2,332 1,959
3.73 GM 12-bolt 3,134 2,350 1,975
3.89 Ford 9" 3,269 2,452 2,059
4.11 Ford 9" 3,454 2,590 2,176
  • TH400 (no overdrive, no lockup): on a 9" the closest options are 3.50 (about 2,950 rpm at 70) or 3.00 (about 2,500 rpm; very tall, only with a 2,500+ stall converter and a torquey cam, and it will feel lazy off the line). For a 35-minute daily interstate run, about 2,950 rpm with a lopey 454 is loud and is the honest weakness of the TH400 path; the owner asked to avoid "spun to the moon" rpm, so this is a real argument for the 4L80E. GM 12-bolt: 3.42 is the compromise (about 2,870 rpm), 3.23 is calmer. Forum guidance for 454/TH400 trucks is 3.42 or 3.73 [50] (page blocked; not re-verified).
  • 4L80E: 9" with 3.70 or 3.89 (about 2,330-2,450 rpm at 70); 12-bolt with 3.73. The overdrive makes first gear lively without highway drone. This is the best match for the stated use.
  • T56 Magnum (corrected): 9" with 3.70 (about 1,960 rpm at 70 in the 0.63 sixth at spec diameter, about 2,030 measured) or 3.89 (about 2,060); the earlier 3.89-4.11 guidance was written for a 0.50 sixth that no longer ships. 3.50 would make launches lazy and 6th gear lug. Ratio feel is the owner's preference; a Ford 9" third-member swap is about an hour.

Why it matters

The rear axle is the one driveline part that sees all of the engine's torque multiplied by first gear and the converter, and it is the part most likely to break first when a 454 gets built. A 1977 C10 Bonanza with the "heavy half" / Big 10 package most likely has the GM truck 12-bolt (8.875" ring gear, 30-spline C-clip axles); period "Big 10 454" truck listings commonly show a 12-bolt, but a standard-spec C10 often has the 8.5" 10-bolt instead, so verify (10 cover bolts vs 12). If it turns out to be a 10-bolt, skip the 12-bolt rebuild path entirely (the parts below do not fit it) and go straight to the 9". GM used the truck 12-bolt in half-ton trucks from 1964 into the early 1980s [7]. Forum members disagree about when it ended (some say 1979, some 1981, and one thread has a 1985 truck whose 12-bolt may have been swapped in) [1][2][3]. It is a good axle for a stock 454. It is not an ideal axle for 500 hp, 550 lb-ft, sticky 295-width tires and a daily-driven teenager's truck in 2042.

The honest strength picture: forum consensus is that a factory 12-bolt "will hold 400-500 hp" and that traction is what actually breaks parts [4]. One owner who broke a 12-bolt three times behind a big block with 35s said he would not put money into one [5]. The weak links are the C-clip axle shafts (a broken shaft can let the wheel leave the truck), the carrier, and the lack of a pinion support bearing (the 12-bolt has no pinion pilot/"Daytona" support like a built 9"), not the ring gear. Path B (about 400-450 hp, street tires) is a sensible fit for a rebuilt 12-bolt with upgraded axles. Path A (500+ hp, 540+ lb-ft) wants a Ford 9" or Dana 60. Because this is a decade-long build that ends as a teenager's car, I recommend paying once for a retained-axle, parts-everywhere Ford 9".

Two truck-specific wrinkles. First, an unmodified 2WD '73-87 C10 is 5-lug 5x5" [6]. Your "heavy half/Big 10" package could be 6-lug, so check the hubs first, because every axle and brake option below is lug-pattern specific. Second, the stock driveshaft/slip yoke spline count (TH400 is 32-spline) must be read off the truck. Any trans change (4L80E or T56) means a new driveshaft anyway, so the axle choice and the transmission choice should be made together.

Options

0. First, identify what you have

Check How Source
Truck 12-bolt vs car 12-bolt Cover has 12 bolts and an irregular shape with a raised ring-gear bulge; the car cover is round. Parts are not interchangeable (carrier housings differ) [7][8]
Ratio and axle ID Stamped on the front of the passenger-side axle tube; on 12-bolts the stamp sits partway between the shock tab and the casting number; the pre-1981 part number "3896839" is seen near it [3] [3]
RPO codes Service Parts Identification (SPID) label in the glovebox (or door); axle codes start with F, G or H; G80 = limited slip/Positraction; a separate code gives ratio (examples seen: GT5 = 4.10, GU6 = 3.42). Only valid if the axle was never swapped [9][10]
Turn count Jack both rear wheels up, mark the driveshaft and a tire; turn the shaft until the tire makes one turn (open diff: turn one wheel with the other held). Count of shaft turns = ratio. Pull the cover to read the tooth counts on the ring (e.g. 41/11 = 3.73) for certainty general practice (confirm by tooth count)
Differential type G80 on the label, or spin one wheel with the other held: same direction means posi [9]

1. Rebuild the GM 12-bolt truck axle

Gears (8.875" truck, "thick ring gear" matters on 30-spline carriers):

Tier Brand and model Part # Price Link Notes
Good Motive Gear 8.875" 12-bolt truck 3.73 683-GM12-373 not confirmed (JEGS listing exists) [11] Check the listing is truck, not car, before buying
Good /Better Richmond Excel GM 8.75/8.875" 12-bolt truck 3.73 836-12BT373 $223.99 (JEGS search snippet, rechecked 2026-10-12; 41/11 teeth, 30-spline pinion; page itself not fetchable) [12] Cheapest name-brand truck set found
Better Yukon Gear high-performance ring and pinion, GM 12-bolt truck YG GM12T-373 (JEGS) / YG GM12T-373T (Denny's, thick ring) $411.99 (JEGS snippet); Denny's YG GM12T-373T $280.98 but listed "Temporarily Not Available" on 2026-10-12 [13][14] Ratios 3.08 to 4.56 exist
Best Same, Yukon "USA Standard Gear" installed kit with Posi, bearings 10688 (3.73 install kit, 30-spline Posi) price not confirmed (JEGS page blocked) [15] Includes bearings and seals; verify what is in the box
Overhaul kit Yukon Master Overhaul Kit, GM 12-bolt truck, Timken YK GM12T-FDHC $318.89 (completewebsol.com) [16] Bearings, shims, seals, bolts, crush sleeve

Carriers:

Tier Brand and model Part # Price Link Notes
Good Yukon Dura Grip clutch posi, 12-bolt truck 30-spline, 3.73 and up YDGGM12T-4-30-1 $919.89 (liftkits4less, fetched 2026-10-12); the 2.76-3.42 version was ~$415 (snippet only) [17] Clutch type, wears, needs additive fluid
Good Eaton Posi Performance, 12-bolt truck 30-spline 19556-010 $750.99 (Summit snippet) / $679.99 (Mark Williams snippet), 2026-10-12 [18] Original-style clutch posi, cheap and proven
Better Auburn Gear Grip-N-Loc, 12-bolt truck 30-spline, 3.73 up 546033 (per JEGS snippet; not independently confirmed) $520.99 (JEGS snippet, 2026-10-12; earlier $609.90 figure was stale) [19] Cone-style, good street manners
Better Eaton/Detroit Truetrac (helical), GM half-ton truck 12-bolt, 30-spline, 3.73 and up 913A315 $725.19 sale (Fass Motorsports) to $764.26 sale (ThreePiece.us), search snippets 2026-10-12; one retailer listed $1,045.99, so shop around [20a] Exists for the truck 12-bolt (earlier draft wrongly said it did not). The NAH Rods link [20] is the CAR 8.875" unit, 913A555, $716.99, do NOT buy that one for a truck
Best Eaton Detroit Locker (normally-locked, street usable) 187C150A, truck 12-bolt 30-spline, 3.73 up $1,146.99 (Revolution Gear) [21] Clunky on parking-lot turns; wears tires. Not what I would give a teenager

Axles (the real upgrade for a 12-bolt):

Tier Brand and model Part # Price Link Notes
Good Moser 5-lug conversion 30-spline axle set for 12-bolt truck, "late '70 C10" (a 63.625" wheel-mount-to-wheel-mount housing) A30-70GMT2 (per AZ Pro URL slug) $335.00 per pair (AZ Pro Performance, fetched 2026-10-12) [51] Moser says it fits ONLY a 63.625" housing; Currie lists 63.625" as the stock 73-87 width, so it may fit a 73-87 12-bolt, but measure first. Original-listing [22] (a '65-69 set) is a different, unconfirmed product
Better Moser 12-bolt housing and axle package, alloy 30-spline pressed-bearing axles (car, F-body) M12125 $2,023.99 (Classic Industries) [23] Shows the price level for a new nodular housing package; not a C10 fit
Best Moser complete 12-bolt assembly, Eaton posi, pressed-bearing 30-spline axles M12120 $4,174.99 (Classic Industries) [24] Car-based; not a C10-specific part. Moser's truck 12-bolt housing availability for 73-87 is not confirmed

Pressed-bearing (retainer-plate) axles remove the C-clip failure mode; this is the single most important safety upgrade if you keep a 12-bolt.

Rough cost to rebuild: gears $225-410, kit $319, carrier $520-920 (Auburn, Eaton Posi, Truetrac, Dura Grip; a Detroit Locker is $1,147), axles $335-$700, plus seals and hardware: about $1,400-$2,600 for a street-strong rebuild (labor is yours). The earlier $1,200-$2,200 figure was low once current carrier prices were applied.

2. Ford 9" bolt-in (leaf spring) for 73-87 C10 2WD

Tier Brand and model Part # Price Link Notes
Good Strange Engineering 9" housing only, '73-87 C10 HF9LEG73C10ME $895.00 (Strange) [25] Mild-steel, HD leaf and shock mounts; custom width at no charge. Need axles, third member, brakes
Better Currie Centurion 9" housing and 31-spline axle package, 5-lug GMC7387X5 (CJ Pony); AZ Pro lists it as CE-GMC7387X5-1-32; 6-lug is CE-GMC7387X6X $1,750.00 list (AZ Pro fetched 2026-10-12; JEGS snippet); $1,550 sale at CJ Pony / Performance Online (snippet). Note the Performance Online page now defaults to the 6-lug SKU, so pick 5-lug explicitly [26][27][28] Stock overall width 63.625", 0.5" pinion offset, 3" x 0.25" tube, 1/4" retainer plates, OEM-location spring pads; takes drum, Explorer disc or Wilwood disc. Also sold as a complete crate with Currie TwinTrac third member and ratio: CE-GMC7387X5-07, $3,150 (AZ Pro) [28a]
Best Detroit Speed 9" QUADRALink housing (for the 4-link suspension upgrade, not leaf) 073042DS $2,047.95 (Detroit Speed, search snippet 2026-10-12; earlier $1,949.95 figure was stale); floater/C6 versions at CJ Pony [29][30] Only if you also do the Detroit Speed 4-link; far more fab and cost

Third members (the part that sets gears and carrier):

Tier Brand and model Part # Price Link Notes
Good JEGS Ford 9" Posi third member, 31-spline, 3.50 or 3.89 555-60680 (3.50), 555-60684 (3.89) $1,249.99 and $1,270.99 (JEGS snippet, not re-confirmed). JEGS describes nodular case, Daytona pinion support, Trac-Loc, 1310 yoke [31] Budget. Note the 1310 yoke: swap to a 1350 yoke if you run a 1350 shaft. Ratio choice here is 3.50 or 3.89 only
Better Currie 9" 31-spline TwinTrac third member CE-9CT[ratio]S50 pattern (exact number not confirmed) $1,401.99 sale (OPGI) to $1,749.99 (Classic Industries); one eBay listing $1,901.50 for CE-9CT370S (3.70). Snippets 2026-10-12. Currie ratios: 3.00, 3.50, 3.70, 3.89, 4.11 [32] Currie's TwinTrac is a gear-type (Torsen-style) posi; quiet for street. Ford 9" does NOT come in GM ratios like 3.23/3.42/3.73
Better Strange S-Series 31-spline, Trac Lock, 3.50 873-PJ9ST31S350 $1,352.99 (JEGS snippet) [33] Clutch type
Best Strange nodular 9" case with Ultra pinion support not confirmed not confirmed [34] Case alone; a pinion support is the main strength feature of the 9" at 500+ hp

Total package: about $3,000-$3,600 with Currie housing plus a $1,300-$1,750 third member, before rear discs (the Currie complete crate at $3,150 is the cleanest way to price it). Weight and drag: the earlier draft claimed no weight penalty and 8-9 rwhp of drag, which the cited thread [35] does not support. That thread (Camaro forum, car-based, partly disputed by its own posters) says a 9" runs roughly 75 lb heavier than a stock 10-bolt and a 12-bolt about 60 lb heavier, and claims higher drivetrain loss for the 9". By that thread's own numbers the 9" is only about 15 lb heavier than a 12-bolt (my arithmetic; unverified), and neither weight nor drag is a deciding factor for a street truck. Real 9" gotchas: the thread reports the 9" pinion centerline sits about 0.75" lower than the GM unit's (unverified), and Currie lists 0.5" of pinion offset, so confirm driveshaft angle and exhaust/tunnel clearance at ride height.

3. GM 14-bolt semi-float (9.5")

Cheap, easy and strong, but wrong for this truck. The 14-bolt semi-float has larger axle tubes (U-bolt about 3-3/8" versus 3"), so you need new perches, U-bolts and shock mounts, and may need to relocate perches from later donors [36][37]. Forum voices call it better for off-road or heavy towing, and ring and pinion costs and weight are higher. A 1988+ donor is a different animal (wheel offset, brake and ABS options) and I found no evidence it bolts to a '73-87 without fab. Not recommended; price not researched beyond a donor axle (commonly cheap used).

4. Aftermarket complete 12-bolt, or Dana 60

A new Moser 12-bolt assembly is the safest keep-the-12 answer ($2,000-$4,200, car-based part numbers found only). Strange also sells a Dana 60 for '73-87 C10: $2,525 with Pro Race axles, $2,895 with alloy axles and Trac-Lock, housing only $1,110-$1,125 [38]. The Dana 60 has a 9.75" ring gear, better driveline angle and some driveshaft clearance, but it is heavy, and its gearing is less convenient than the 9". It is overbuilt for a street C10.

Rear disc brakes

Tier Brand and model Part # Price Link Notes
Good CPP 5-lug rear disc conversion, 10/12-bolt C10 73-87, 11" drilled/slotted rotor DBR320-V $770.99 (CJ Pony search snippet 2026-10-12; page 403) [39] CJ Pony lists it for 10- or 12-bolt, includes pads and e-brake cables, for non-staggered shock setups only (check your shocks)
Best Wilwood Superlite 4-piston rear kit, 12.88" rotor, C10 63-87 DBR205D $2,141.96 (CJ Pony snippet 2026-10-12; MSRP $2,399; red version DBR205DR same price) [40] Fits C10 with the large Chevy flange per the listing; parking-brake provision varies, confirm
Ford 9" Currie housing ends accept Explorer, POL or Wilwood disc setups n/a see housing [26] Easiest rear-disc path: Explorer 9" disc brackets are cheap

The matching front kit I saw is the Wilwood Deluxe C10 front for $789.23 (CK5-7387WB, fetched 2026-10-12) [41]; it requires the original heavy-duty disc spindles and 1.25" rotors. A drum-front 1977 truck may need spindles, so see the brakes section.

Driveshaft

Tier Brand and model Part # Price Link Notes
Good Denny's Driveshaft custom steel, 3.0-4" tube, 3R/1310/1330/1350, steel or 6061 aluminum custom order price not confirmed (the earlier $775.95 figure could not be re-verified; the page shows no prices; they quote by phone) [42] Ships anywhere; measure per their guide. Specify a 32-spline slip yoke for TH400/4L80E
Better Modern Driveline custom aluminum 3.5" tube, 1350, forged yoke custom (build sheet) $625 forged-yoke or $845 billet-yoke (search snippets 2026-10-12). The $980 figure in the earlier draft was for a Ford 8.8-flange listing and is withdrawn [43] CAUTION: those listings are 31-SPLINE (T56 Magnum/Ford). A TH400 or 4L80E needs a 32-SPLINE yoke, so confirm the build sheet before ordering
Best Tom Wood's custom Chevy rear driveshafts (4xshaft.com) custom 1350-series double-cardan shafts "starting at $604" (page fetched 2026-10-12) [44] Full-custom Chevy rear shafts; complete 1350 street shafts will price higher than the starting figure
Slip yoke Richmond Excel 32-spline slip yoke YK13 price not confirmed (CJ Pony page 403; part number not re-verified) [45] Verify series (1310 vs 1350) before ordering; TH400 and 4L80E are 32-spline
Slip yoke (T56) Sonnax forged chromoly 1350 T3-3-2491HP (TKO 600, T56 Magnum, TKX; 31 spline) $197.88 (Denny's, fetched 2026-10-12) [46] Magnum F models need the shorter T3-3-2491S. Neapco N3-3-4271X is a 27-spline version; check your T56 output spline

Key driveline facts: TH400 and 4L80E share a 32-spline output, so one 32-spline 1350 slip yoke serves either [45]. Tail-housing length (short vs long TH400 case, 4L80E, T56) changes shaft length. Stock '73-87 C10 shafts are 1310 or smaller; go to 1350 for 500 hp. Critical speed: a longer, thinner tube is worse. The critical-speed figures I originally quoted (3.5" steel 35" long about 7,950 rpm; 50" about 7,110 rpm [47]; 4" aluminum about 7,900 vs 3" mild steel 5,800 [48]) came from pages that are now gone (HTTP 410) or were not re-verified, so treat them as ballpark. Have the shaft builder quote the critical speed for your measured length; anything above about 5,500 driveshaft rpm is ample here. Your drive speed at 70 mph is roughly 3,000 driveshaft rpm with a 1:1 TH400 gear, so margins are fine either way; aluminum helps if you go to a long T56 (the shaft is longer) and keeps weight down. Local shops: my search surfaced FleetPride's Kansas City, MO location (1250 N. Corrington Ave., 816-231-8816) as a place that balances driveshafts, but I could not confirm that they build custom car shafts [49]. Mail-order Denny's, Inland Empire or Tom Woods all work, and any KC truck driveline shop can balance and install.

Fitment & gotchas

  • Lug pattern. 2WD '73-87 C10s are 5x5" (5-lug); K/4WD is 6x5.5 [6]. Verify yours. Currie sells the package in both [26][27].
  • Axle width. Currie matches stock width. Currie lists the stock 73-87 overall width as 63.625" [28]. A 12-bolt axle set is length-specific (Moser's late-'70 5-lug conversion axle fits only housings measuring 63.625" wheel-mount to wheel-mount) [51]. Measure before ordering any axles.
  • Big rear tires. Stock width is built for stock offset wheels. A 275-295 rear tire on a 9-10" wheel needs backspacing around 4.5-5" for the 15" class; verify at test fit, and note that Strange offers custom widths at no charge [25]. Confirm the exact numbers against your chosen wheel before ordering the housing.
  • Pinion angle. One owner reported driveline vibration after install because the pinion pointed down about 4 degrees; set pinion angle with adjustable shims or a wedge [52]. Cheap trick: parallel U-joint angles and a measured inclinometer reading.
  • Brake lines and ABS. Floater housings allow ABS and C6 brakes (Detroit Speed) [29].
  • Parking brake. Disc kits that skip the drum-in-hat parking brake need a separate caliper or a drum-in-hat rotor; confirm per kit.
  • C-clip axles. If you keep a 12-bolt, replace C-clip axles with pressed-bearing ones, and keep the cover on a fresh gasket with correct fluid.
  • Driveshaft length. Measure with the axle at ride height, transmission mounted, slip yoke at mid-travel. A new transmission or axle always means a new shaft.

How it touches other systems

  • Transmission: the ratio table above. 32-spline slip yoke for TH400/4L80E, 31 spline for T56 Magnum (27 spline on some other T56 variants) [45][46]. Tail-housing length changes shaft length.
  • Engine and converter: a 3.73 to 3.89 gear with a 3,000 rpm-stall converter and a lumpy cam can make the truck feel slow off the line; a gear change is cheaper than a converter change.
  • Wheels and tires: gearing is tuned to the tire diameter; a 30" tire needs about 7 percent more gear than a 28" for the same cruise rpm.
  • Brakes: rear discs change the proportioning; a residual valve or adjustable proportioning valve, and a booster/master-cylinder matched to front/rear discs, are needed.
  • Suspension: a 9" housing bolts to leaf springs; the Detroit Speed QUADRALink only makes sense with a full 4-link rear suspension. A drop (lowering blocks, flip kit or lowering springs) interacts with pinion angle and driveshaft length.
  • Exhaust: a dual 3" system with an X-pipe must clear the driveshaft and rear tunnel.
  • A/C: no direct effect.

Check on the truck

  1. Pull the diff cover. Count the bolts, read the ring tooth counts, and photograph the casting numbers. Is it a truck 12-bolt (irregular cover) as assumed?
  2. Read the axle-tube stamping and the glovebox SPID codes (G80 etc.). Is there a posi?
  3. How many lugs on the rear hubs: 5-lug or 6-lug? What is the wheel-mount to wheel-mount width?
  4. What is the stock driveshaft U-joint size and slip-yoke spline count (27 or 32)?
  5. Do you plan the 4L80E or T56? That sets the 32-spline vs 31/27-spline yoke and the final-drive ratio.
  6. How wide a rear tire do you want, and what wheel width and backspacing?
  7. Are you willing to spend about $3,000-$3,600 now for the 9" (Currie crate $3,150 as of 2026-10-12)? Or do you prefer to stay with the 12-bolt for Path B?
  8. Is the rear axle actually a 12-bolt (Big 10 package) or the standard 8.5" 10-bolt? Count cover bolts first.
  9. Are rear discs required for the parking brake, or is the stock parking brake ok?

Fact-check notes

Adversarial check, 2026-10-12. Fetched live (OK): Currie package at AZ Pro Performance and Performance Online, Currie crate (AZ Pro), Strange 9" housing and Strange C10 rear-end index (Dana 60 prices confirmed), Revolution Gear Detroit Locker ($1,146.99, 187C150A), Yukon overhaul kit ($318.89), Yukon Dura Grip, Denny's Yukon gears, Sonnax yoke, Wilwood front kit ($789.23, CK5-7387WB), NAH Rods Truetrac, Moser axle at AZ Pro, ThreePiece bolt pattern (2WD C10 is 5x127, confirmed), dieselhub (12-bolt in trucks 1964-1982), differentials.com RPO codes (G80, GT5=4.10, GU6=3.42 confirmed), Tom Wood's page. Blocked (403/410): CJ Pony, Classic Industries, Summit, JEGS, ck5, stangnet, streetmusclemag, Detroit Speed (prices from search snippets only).

Changes made:

  • Gear ratios (major): a Ford 9" does not come in 3.23/3.42/3.73; Currie lists 3.00/3.50/3.70/3.89/4.11. Recommendation, rpm table and per-transmission advice rebuilt around real 9" ratios.
  • Truetrac: removed the claim that no truck 30-spline Truetrac exists; Eaton 913A315 (truck 12-bolt, 3.73 up) is about $725-764.
  • Prices updated: Dura Grip $919.89 (was ~$800), Auburn $520.99 (was $609.90), Eaton Posi $750.99/$679.99, Detroit Speed 073042DS $2,047.95 (was $1,949.95), Currie 5-lug part number clarified (and Performance Online's page now shows the 6-lug SKU), added Currie crate $3,150.
  • Driveshaft: the $980 Modern Driveline price belonged to a Ford 8.8 flange listing; replaced with $625/$845. Added warning that 31-spline shafts do not fit a 32-spline TH400/4L80E. Denny's $775.95 withdrawn (unverified). Sonnax yoke price added ($197.88). Tom Wood's starting price added.
  • 9" weight/drag claim: the source thread does not say what the draft claimed; rewritten.
  • 12-bolt cost range raised to $1,400-$2,600; Path B 12-bolt kept as a legitimate option; added a 10-bolt caveat (the truck may not have a 12-bolt).
  • Flagged TH400 at about 2,950 rpm at 70 as a weak fit for the "no drone" commute goal, which strengthens the case for the 4L80E.
  • Unverifiable and left as such: JEGS part-number pages, Summit 19556-010 page, Strange 873-PJ9ST31S350, the critical-speed numbers, and the FleetPride KC claim.
  • Removed an unsupported "long-tail TH350/A/C truck" aside.

Sources 54

  1. gmsquarebody.com (How to identify axles)
  2. gmsquarebody.com (12 bolt ID numbers location; found via search)
  3. gmsquarebody.com (12 bolt ID numbers, search result)
  4. gmsquarebody.com (Stock rear-end, how much power)
  5. gmsquarebody.com (454 with 10 bolt rear end?, search result)
  6. threepiece.us (C10 wheel bolt patterns, search result)
  7. dieselhub.com
  8. speedwaymotors.com (search result only; page did not load)
  9. differentials.com (GM RPO codes)
  10. crawlpedia.com (search result)
  11. jegs.com
  12. jegs.com
  13. jegs.com
  14. dennysdriveshaft.com
  15. jegs.com
  16. completewebsol.com
  17. liftkits4less.com
  18. summitracing.com
  19. jegs.com
  20. nahrods.com (fetched; this is the CAR 8.875" Truetrac 913A555, $716.99, not for a truck)
  21. 20a. fassmotorsports.com (search result; Eaton 913A315 truck 12-bolt Truetrac)
  22. revolutiongear.com
  23. jegs.com
  24. classicindustries.com
  25. classicindustries.com
  26. strangeengineering.net
  27. cjponyparts.com
  28. performanceonline.com
  29. azproperformance.com (fetched; CE-GMC7387X5-1-32, $1,750, 63.625" width)
  30. 28a. azproperformance.com (fetched; CE-GMC7387X5-07, $3,150 complete crate)
  31. detroitspeed.com
  32. cjponyparts.com
  33. jegs.com
  34. opgi.com
  35. jegs.com
  36. strangeengineering.net
  37. camaroz28.com (fetched; says 9" ~75 lb over a 10-bolt, 12-bolt ~60 lb, disputed; does not support "175 vs 185 lb" or "8-9 rwhp")
  38. ck5.com (HTTP 403, not re-verified)
  39. ck5.com
  40. strangeengineering.net
  41. cjponyparts.com
  42. cjponyparts.com
  43. performanceonline.com
  44. dennysdriveshaft.com
  45. moderndriveline.com (search result; $625 forged yoke). The previously cited "-88-" URL is a Ford 8.8 flange listing and was replaced
  46. 4xshaft.com
  47. cjponyparts.com
  48. dennysdriveshaft.com
  49. streetmusclemag.com (HTTP 410 Gone on 2026-10-12; figures unverified)
  50. gmsquarebody.com
  51. lmperformance.com (search result; FleetPride KC details came from the search summary)
  52. ck5.com
  53. azproperformance.com
  54. stangnet.com (HTTP 403, not re-verified)
  55. gmsquarebody.com
  56. enginebuildermag.com (search result only)