OBS Chevy Truck Build Planning Guide: Evaluation, Phase Order & Budget Breakdown

The OBS GMT400 (1988–1998) is one of the best build platforms ever made — simple enough for a weekend DIY builder, capable enough for a serious restomod, and with enough community support that nearly every question has already been answered on the FullSizeChevy forum. This guide is your build roadmap: how to evaluate what you’re starting with, how to prioritize your build phases, and how to avoid the money-wasting mistakes that slow most builds down.

Step 1: Evaluate Your Truck Before You Spend a Dollar

The single most important thing you can do before planning an OBS build is a complete, honest condition assessment. Most builds stall or go over budget because the builder didn’t know what they were starting with. Here’s the full evaluation checklist:

Frame Inspection

Get the truck on jack stands and look at every inch of the frame rails. Check:

• Frame rails at all eight body mount locations — this is where they rust through most often
• Front crossmember (between the front wheels) — look for cracks and rust-through
• Rear frame sections from the rear axle to the bumper — these trap water and rust from the inside
• Any prior weld repairs — an amateur weld on a frame is more concerning than rust, as it may have weakened the original section

A compromised frame is not necessarily a dealbreaker — professional frame plating is available — but it changes the budget significantly. A clean frame is worth a significant premium on an OBS truck purchase.

Body and Cab Assessment

• Cab corners (must check): Bottom rear corners of the cab. Surface rust is expected; rust-through that reaches the inner structure changes your metal repair budget by $500–$2,000 per corner at a body shop.
• Rocker panels: Probe with a screwdriver at the center of the rocker. Surface rust is acceptable. A soft spot or hole means inner rocker damage.
• Firewall: Check behind the front wheels for rust — water gets trapped in the inner fender area and attacks the firewall from behind.
• Floor pans: Pull up the carpet completely. Any rust-through in the floor is a structural and safety issue.
• Bed floor: Lift the bed mat. Holes in the bed floor or rotted bed crossmembers add $400–$1,500 to the build depending on replacement approach.

Drivetrain Assessment

• Does the engine start and run? Cold start is the most revealing — if it smokes white from the exhaust (coolant burning), or blue/gray for more than 30 seconds (oil burning), note it.
• Does the transmission shift through all gears? TH350, TH400, and 4L60E all have distinct failure modes — test drive before making any other assessment.
• Are there leaks? Consistent oil leaks on a high-mileage OBS engine are expected. Budget for rear main seal, valve cover gaskets, and oil pan gasket at minimum.
• Does the 4WD engage (if equipped)? Test both high and low range. NP241 transfer cases are common — common failure is a cracked front output shaft seal.

Step 2: Choose Your Build Level

Before ordering a single part, define your build level. This decision determines every other decision — your budget, your timeline, your parts sourcing strategy, and your shop vs. DIY split.

Level 1: Driver Build ($3,000–$8,000 into the truck)

Goal: Mechanically reliable, drives well, looks reasonably clean. No trailer queens here — this truck gets driven. Approach: fix all safety items (brakes, tires, ball joints), address rust at the structural level (frame plating if needed, cab corners patched and painted), make the engine and transmission right, and drive it. A well-executed driver build on a clean OBS is one of the best-value trucks on the road.

Level 2: Restomod ($15,000–$40,000 total)

Goal: Classic OBS body, modern performance and reliability. Core components: LS swap (5.3L LM7 or 6.0L LQ4), 4L60E or T56 6-speed, disc brakes on all four corners, upgraded front suspension (tubular control arms or a subframe swap), modern interior elements (new headliner, seat reupholster or swap, updated gauge cluster or overlay). This is the most popular OBS build path — it produces a truck that looks exactly right but drives and stops like a modern vehicle.

Level 3: Show / Full Restomod ($40,000+)

Goal: Magazine-quality finish. Everything is done to show spec — custom chassis, custom interior, billet everything, paint correction at every step. These builds take 2–5 years part-time and typically end up at specialty shops for the final 30% of work. The market for excellent OBS restomods has strengthened significantly — well-documented $50K+ builds are selling for $60K–$90K at auction.

Step 3: Build Phase Order

The most common build mistake is doing cosmetic work before the mechanicals are sorted. Here is the correct phase order:

1. Rust and structure first: All frame work, cab corner repairs, rocker panel work, and floor pan patches happen before anything else. You can’t paint over rust and call the build done.
2. Drivetrain: Engine swap or rebuild, transmission rebuild or replacement, rear axle inspection and service. Get the truck running right before anything cosmetic.
3. Suspension and brakes: Front suspension rebuild or upgrade (tubular control arms if doing a restomod), disc brake conversion front and rear, new front wheel bearings, alignment. This determines how the truck handles — don’t skip it.
4. Fuel system: If doing an LS swap, upgrade fuel system to EFI-capable (in-tank pump, correct pressure and volume). If keeping the factory 5.7L TBI, service the injectors, replace the fuel filter, and inspect the fuel pump.
5. Electrical: For LS swaps — standalone harness install (see our OBS LS Swap Wiring Harness Guide). For factory engine trucks — address any known wiring issues, replace bulkhead connector if corroded.
6. Body work and paint: All metal work complete and primed before paint. Block sand the panels. Seam seal everything. Spray in a controlled environment.
7. Interior: New carpet, headliner, seat work. Don’t install new interior over a cab that hasn’t been dried out and treated for rust.
8. Wheels, tires, final detail: Last — wheels and tires go on after the build is complete, not before, so you don’t scuff them during the hard work phases.

Step 4: Parts Sourcing Strategy

• Mechanical components (engine, trans, axles): Source from salvage yards and forum classifieds. A 5.3L LM7 truck core from a wrecked 2002–2006 Silverado runs $400–$800 with accessories. Buy from a reputable yard with a return policy.
• Body panels: AMD (Auto Metal Direct) and Goodmark are the reference brands for OBS replacement panels. Quality is OEM-comparable. Avoid no-name Chinese panels — they require significant fitment work that costs more in labor than the panel savings.
• Suspension and brakes: Moog for ball joints and tie rod ends. Bilstein 5100 for shocks. Raybestos or Centric for brake hardware. Don’t cheap out on safety components.
• Interior: LMC Truck is the primary OBS interior parts source. Seat covers, carpet, headliner — all available. For a restomod with a different interior direction, custom upholstery shops or donor interiors from GMT800 trucks (seats swap in with minor modification) are popular options.

OBS Build Resources on FullSizeChevy.com

• OBS / GMT400 Buyers Guide — finding the right truck to start with
• Complete LS Swap Guide — engine selection, mounts, trans pairing
• OBS LS Swap Wiring Harness Guide — standalone harness, tach fix, fuel system
• LS Swap Cost Breakdown — real-world budget numbers
• Best Tires for OBS Trucks
• Modernize Your OBS — tech upgrades that preserve the OBS character

Keep going: find LS swap kits, wiring, costs and engine picks in the LS Swap Hub and the OBS / GMT400 Hub.