What 4-speed manual conversion kit or parts fit behind an LS in a C3 for up to 400 HP?

The simplest route is an LS adapter bellhousing with the GM Muncie pattern. It lets you run a GM 4-speed like the Muncie behind the LS, and later bolt up a GM-spec T5, TKO or TKX with the same bellhousing.

Why you can't just bolt the old bellhousing on: an LS and a small-block Chevy share the bellhousing bolt pattern, but the depth is different, so an old small-block bellhousing puts the transmission in the wrong place. The fix is a bellhousing built for LS dimensions.

We don't have a verified torque rating for the Muncie, so we can't confirm it at 400 hp. If you want overdrive and a known rating, the Tremec TKX 5-speed is rated at 600 lb-ft and is the slimmest 5-speed for tunnel clearance.

You'll also need a clutch, a flywheel and a hydraulic release bearing; kits bundle the release bearing, bellhousing bolts and clutch.