How strong are 8 1/4s

An 8.25 rear with a sure-grip is a fairly strong unit. At a range of 300-350hp(pushing it)- you will be OK.

If you have to have an 8.75 rear, get one from a b-body and move the spring perches. 8.75's are stupid expensive to build and sure grips cost a mint!!

To be honest, a 7.25 sure-grip is quite strong. Just don't expect it to hold up to you're 408 stroker lololol...