horse power VS 8 1/4 rear end

How much horsepower will a 8 1/4 rear take? I have an 8 1/4 rear in the car now and the car came with a new sure grip unit. I'm debating on whether to install the the sure grip along with lower gears or switch to 8 3/4 rear. The car has a 380 horse crate engine. I figure I can probably set up the 8 1/4 for $400 - $500 with new gears and bearings. I'm probably looking at twice that to gather parts and assemble a 8 3/4. I want to stay with the large bolt pattern axles.