The 1988–1998 OBS (Old Body Style) GMT400 Silverado/Sierra is a popular LS swap platform. The engine bay is spacious, the truck is plentiful, and the result is a very capable build. Here’s how to do the engine mounts and crossmember right.
Why Swap an LS into a GMT400?
- The LT1 and TBI engines in these trucks are aging with high mileage
- LS engines are cheap, plentiful, and make great power
- The GMT400 chassis is extremely strong for towing and off-road
Engine Mount Options:
For 2WD GMT400:
2WD 1988–1998 OBS Chevy Truck – LS Swap Engine Mount Kit – Weld-in motor mounts that position the LS correctly in the factory SBC/LT1 location.
For 4WD GMT400:
A 4WD-specific mount kit is needed to clear the front differential and CV shafts. The 4WD mounts position the engine higher and further back.
Crossmember Considerations:
The LS engine typically uses the stock crossmember location with some modifications. A custom or universal LS crossmember kit may be needed depending on transmission choice.
Transmission Pairings:
- 4L60E: Factory transmission in most GMT400 trucks – may bolt up directly with an LS
- 4L80E: Better for high-power or heavy-tow builds
- T56 Manual: Requires custom tunnel work
Engine Fitment Options:
- LM7 5.3L: Cheapest, most plentiful, iron block
- LQ4 6.0L: More torque, good for towing builds
- LS1 5.7L: F-body engine, aluminum block, rev-happy
- LQ9 6.0L: Aluminum heads on iron block, excellent combo
The Complete Swap Parts List:
- Engine mounts
- Standalone wiring harness
- EFI fuel pump and fuel system upgrade
- LS-compatible oil pan (may need truck pan from donor)
- Accessory drive bracket kit
- Exhaust headers
What LS engine did you choose for your GMT400 swap? Share your setup below!