If there is a Mount Rushmore of American trucks, the 1973–1987 Chevrolet and GMC “Square Body” is sitting right in the middle of it. They are big, blocky, and built like bank vaults.
But let’s be honest: the original 350 small blocks and wheezing emissions-choked V8s from the late 70s and 80s aren’t exactly thrilling. They leak oil, they hate cold mornings, and getting decent horsepower out of them takes serious cash.
The solution? The LS Swap. Taking a modern fuel-injected GM LS engine (like a 5.3L or 6.0L out of a newer Silverado or Tahoe) and dropping it into a classic Square Body is the single best upgrade you can do. You get 300+ horsepower out of the box, it starts every time you turn the key, and you can still drive it across the country.
If you are a beginner looking for your first major engine swap, this is the chassis to learn on. Here is the reality of what it takes to get it done in your driveway.
1. The Engine Bay is a Cavern
The biggest headache of engine swapping is usually space. Not here. You could practically stand inside the engine bay of a C10.
Because there is so much room, you don’t have to fight to make things fit. You can keep the truck accessories (alternator, power steering pump) exactly where they are on the donor engine, which saves you hundreds of dollars on custom bracket kits.
2. The Mounts Make it Easy
You don’t need a welder to get the engine sitting in the frame. The aftermarket support for Square Bodies is massive. You can buy a set of adapter plates for fifty bucks that bolt directly to the LS block, allowing you to use the original old-school rubber clamshell motor mounts already sitting on your frame rails.
3. The Big Three Hurdles (And How to Jump Them)
While it is an easy swap, it’s not just “plug and play.” Here are the three things you have to tackle:
• The Oil Pan: The deep, ugly oil pan from a modern truck LS engine will hang down below your Square Body’s crossmember. If your truck is lowered, you will crack it on a speedbump. You’ll need a retrofit swap pan (like a Holley) or an F-Body (Camaro) pan for clearance.
• The Fuel System: Old carburetors run on 5 to 7 PSI of fuel pressure. A modern LS needs 58 PSI. You have to upgrade your fuel lines and run an EFI fuel pump. The easiest beginner route is an inline pump mounted to the frame rail, or dropping a modern sending unit right into your factory saddle tanks.
• The Wiring: This is what scares beginners the most, but it shouldn’t. You can send your junkyard wiring harness off to be “thinned out,” or buy a standalone harness. You hook up a few power wires, a ground, and a keyed ignition wire, and the computer does the rest.
The Verdict
An LS-swapped Square Body gives you the aggressive, classic styling of the 70s and 80s with the reliability of a modern daily driver. It is the perfect project to learn how fuel injection works, how to route plumbing, and how to build a truck that actually runs as good as it looks.
Stop staring at that empty engine bay. Go hit the junkyard, pull a 5.3, and get to work.