I ran a '73 Monte Carlo with a 400 hp 350 +.060 with a stock TH350 and a shift kit. Never had a problem with the trans. Blew up the 10 bolt rear once though. I did not run an external cooler but I did use a 4 core radiator. Make sure it is running at normal temperatures (~185-190F). The TH400 has higher internal losses than the TH350.

Kitz