Should I LS-swap my SBC 400 squarebody or keep the 400?

Speedway's comparison focuses on the 350 vs the 5.3 and skips the 400 on purpose, so we can't give a head-to-head number for the 400. The general differences still apply:

	Small-block Chevy	LS
Valve angle	23° (stock heads)	15°, cathedral or rectangle port
Cam	Flat tappet or roller	Roller only
Oil pump	Distributor-driven, rear pickup	Crank-driven gerotor, front pickup
Ignition	Distributor	Coil-near-plug, no distributor
Bottom end	Many older blocks are two-bolt main	Cross-bolted mains
Dressed weight (Speedway)	about 575 lb	about 430–470 lb (aluminum)

Speedway credits the small block with strong low-end torque — a reason to keep it; the LS's heads, oiling, lighter weight and EFI drivability are the reasons to swap. If the 400 is healthy and you like how it drives, keeping it is reasonable. If it needs a rebuild anyway, compare that cost against a junkyard 5.3 or 6.0 with the swap cost estimator — a 6.0 is the closer displacement match.

If the truck or car already had a small block, the LS drops in with a handful of swap parts: LS-specific motor mounts (the bolt pattern is different), an oil pan that clears the crossmember, and either your GM transmission with an adapter or an LS-pattern transmission — the bellhousing patterns differ slightly. EFI also needs a harness, a computer and a high-pressure fuel system.