LS3 (hot cam LS376/480) into an '85 IROC — which manual transmission is strong enough, and what LS3-specific swap issues exist?

Transmission: a stock LS3 already makes about 424–428 lb-ft, and a hot-cam crate engine makes more, so a T5 is out.

Transmission	Torque capacity	Notes
Tremec TKX (5-speed)	600 lb-ft	Slimmest 5-speed for tunnel clearance; 3 shifter locations; most popular LS swap manual today
Tremec Magnum (6-speed)	700 lb-ft	T56-family size; multiple shifter locations
T5 World Class	300 lb-ft	Standard/non-World Class 265 lb-ft

The TKX fits this well: 600 lb-ft capacity and the slimmest case for tunnel clearance. A T56/Magnum-size 6-speed has more capacity, but Speedway notes the T56's size often means firewall and tunnel modifications.

Bellhousing: the steel LS bellhousing (GM pattern) takes the TKX; T56/TR6060 units need their own LS bellhousing. You'll also need a clutch, a flywheel and a hydraulic release bearing; kits bundle the release bearing, bellhousing bolts and clutch.

We don't have sourced LS3-specific third-gen fitment notes; use the planner for chassis parts.