How big of tires are you running? That is probably going to make more of a dent in your axle then just a big engine.
I knew a guy that dropped a 454 in a Chevy Luv (They had an engine and transmission... and a luv missing both in the back) and the rear axle handled the horsepower fine. Then again, it's not like it could keep traction long enough to put any power down...