Suspension, stance & handling

Pick: the "Better" path, built in stages, targeting about 3.5" front / 5" rear.

1. Keep the factory-rate big-block front spring (Moog 6454-type, 1,060 lb/in) and drop with spindles, not with a soft or cut spring [8][14]. A heavy-half 454 truck sits high and heavy on its nose, and lowering springs rated for a small block can ride poorly or bottom. A 2.5-3" spindle plus (if needed) a 1" drop spring gets close to the owner's "3.5 front" target without altering geometry. This matches the owner-reported 3.5"/5" build [15] (2.5" spindles + 1" drop coils on a small block). With a 3" Belltech spindle and stock-height 6454 springs expect about 3" up front; add a 1" drop coil only if the 454 truck does not sit as low as wanted, and note a lowering coil is usually a softer rate than the 6454. Whether the 6454 is actually in the truck needs confirmation; see questions.
2. Spindle choice depends on the brakes the truck actually has: Belltech 3100 for 1.25" rotors vs. 3200 for 1.0" rotors [3][4], or other brands' equivalents. This is the most expensive mistake to make. Light-duty and heavy-duty front brakes use different spindles/calipers and cannot be mixed [41]. The owner says "drums all around", but 1971+ C10s were built with front discs, so this is almost certainly a mis-remembered detail; look at the wheel. Source [9] only confirms J55 heavy-duty power brakes and larger rear drums for F44, not the front rotor thickness. One retailer lists the Belltech 3200 as "heavy duty brake w/ 1 in thick rotor" while Belltech's own page calls 3200 light duty; measure the rotor rather than trusting names. Identify before buying.
3. Rear: a 5" flip kit plus a bolt-in C-notch plus relocated or proper-length shocks. It is the cheap, serviceable, leaf-spring approach. Expect some ride harshness because the stock heavy leaf pack (if it is the F44 8-leaf) is stiff and sits under bump stops sooner. A reasonable alternative for a commuter and teen-driven truck is a fresh lowered leaf pack (Performance Online RLS7387S6, 3" or 4" drop, $895.74) with optional Detroit Speed drop shackles, which avoids the C-notch and pinion-angle changes of a flip but gives less drop. If the flip is too stiff or low, step to the 4-link later.
4. Shocks: Bilstein front 55-R033 plus rear B6 for the 6" lowered rear (or the 244131 kit if only about 5" is wanted) [13][27][28]. Monotube gas shocks give controlled ride at 70 mph.
5. Sway bars: a 1-3/8" front (UMI or Belltech 9908) and a 1" to 1-1/4" rear (QA1) is plenty for a 454 street truck.
6. Refresh everything now: ball joints, tie rods, idler/pitman arms, control arm bushings (poly or rubber), steering box inspection, cab mounts, springs. The truck sat about 10 years.

If the owner wants one purchase and done: UMI 6404 Stage 4 at $4,728.99 [30], or Ridetech's coilover/4-link if the owner wants a pro-touring truck [10]. These cost far more than the "Better" path and bring more adjustability than a 2042 hand-me-down truck needs.

If the owner decides on 17" wheels and wide rears: the 4-link path gains rear room, but the spindle-and-flip route still works with 255/275-width tires; forum members like 255/60 front and 275/60 rear on 15x8 wheels for lowered trucks [43][42].