What would need to be changed in a turbo 350 transmission to make it live behind a 455 with 466 hp and 500 torque? Will be street driven and occasionally taken to the strip for test and tune night. M/T 9" Street ET's 3:42 rear end.

 

Not much if the trans is in good condition. At the absolute minimum, install a shift kit, preferably one that uses a aftermarket pressure regulator spring. The Trans-Go shift kit, the B&M Transpak, and the TCI Trans-scat kit do. If your overhauling the trans, machine the direct clutch piston to allow installation of an added clutch and steel. A aftermarket intermediate sprag race would be a nice upgrade, but make sure you get a quality part! Some of the cheap aftermarket races are much harder (weaker) than stock, and will shatter. Trans-Go, Coan, etc all have good quality parts.

I helped coppercutlass a few weeks ago with his TH350 build, read that thread for some more build tips.