How atrong are the rear-ends in these cars. I know the L-88s had alot of power, but most people tell me that after 400hp, stuff breaks. What is the limit, and how would you beef em up?

This is a great question ! The only thing I know of is to replace the greasable u-joint with a pre-greased solid u-joint. Everyone keeps telling me that I will break rear suspension components with my new engine if I do hole shots but they only thing they seem to be able to offer is to change the u-joint. It is possible to change the whole rear suspension over to a solid axle, but that does not seem to be in any way cost effective. It seems a real problem is the half shafts actually twisting from torque until they break.
I would like to hear some "easy" ways to beef up the rear suspension for straight line runs also.

Didn't the big-blocks have heavier half-shafts?

I think if you want to upgrade you can put the non-greasable u-joints in. Also , on the older vettes(1963 to 1979???) you can put in a 7 leaf rear spring, which is the heavy duty one or go to a fiberglass one.