my father and i own a transmission shop, and from what ive seen its pretty impossible to answer. yes some fo the earlier th400's like in a long tailed caddy seem to somewhat have some extra beef in the case. and in every severe duty application we have always used the 8 bolt pump's. but never had a non 8 bolt fail(other then it beign a rebuilders fault). im unsure how early back it will go but some cases are molded with a "HD" on the bell. id assume it meant heavy duty. the seem to come in truck applications both 2wd and 4wd. iv used both cases and never saw any hard proof that one is really better then another. i had a case stamped "HD" in my camaro and blew the case apart after a season with street miles, and have a non hd case for 3 or 4 seasons with alotta abuse/passes/streetmiles and still holds strong. both transmissions are set up similar. this is with a 3800lb camaro in the low 9's on spray. not as fast as most of you guys but at 3800lbs the titanic wasnt fast neither.
there is only one better case that i have seen that i could say is really overall better, and it is super rare. some cases and a set screw to hold the center support. its help alot but are almost impossible to find these days.