8-1/4" rear end questions?

I have read a couple of time that the 8-1/4" would be good for about 400hp... and I alway wonder what is the weak point of that rear-end?
Is it the axles? The gears? Both?

I'm just curious...that's what I have in my Dart now, and I am still very very very far away from the 400hp (72 318 2 bbl all stock, almost :sleepy1: ). Power should increase in the next years, don't know if I will it 400hp one day...I wish.

Thanks