My Trans builder here in Oregon (he is a bracket racer) told me that a good built TH350 is only acceptable to about 450HP. He will not build one if the application is over that. TH400 is the way to go.
We have a t-350 behind a bbc running 5.9's 1/8th in a 3200-3300 #'s firebird. We had another one in a bbc 3320# car running 6.2's & 30's. They will live with good parts (or atleast they have for us).
Lanes Automotive in Buffalo,MO built ours. 417-345-6725