a 350 taking 30% of your horsepower ? that seems kinda high, were did you get that from, not saying it cant be right but ??? . anyway my brother had a 69 chevelle with about 375 hp and he put 40K on so far without any tranny problems. a 400 is definatly stronger but if the right guy ( my brother in law ) builds the tranny to suit the need, i dont think you will have a problem, not right away anyhow