Somewhere in the area you are talking, north of 500 horse, the case will flex. They used to put a girdle on them where the pan bolts up, but not sure if that "fix" worked or is still used today.

Check outfits like this for the list of parts they add for high power applications:

TCI Auto: Super StreetFighter™ Transmission; '66 & Later FE C6 13 1/2" Tailshaft (332/352/390, 406, 427, 428)