Everything. Input shaft, converter hub, OD planet and clutches were always good and beat up, so it's not like line was crazy high and clutches weren't getting their share, lol. If the line was any lower the clutches would just smoke in no time.
A trans of that size can only shift under that kind of torque so many times before the splines rip off of something, it cracks in half or a shaft twists off. I NEVER took the trans out that the input shaft was not bent. NEVER... Always had to work my ass off to get it out of the trans. If you looked down the splines they would run perfectly straight then twiiiiist, lol.
I take that back.... the time the input shaft exploded and went through the pump snout that one wasn't bent
Trans failure is a game of service life vs stress. You can stress it hard at 1000+hp bouncing down the track and fail it in a number of minutes, or stress it moderately at 600+ and fail it in months. Fwiw, I never had a failure sub 500rwhp. Must be the magic point where forces start pushing parts past their yield.