How much can the rear support?

Hello out there. I have a Chevrolet PU 2wd and a 350 motor out of a late 70's caprice so I'm told. One day I tried to do a burnout and only one tire spun. The best way to fix this is to get a 3.73 posi? My other question is once I buy a 12 bolt, I would like to put in a 454 or 502 HO. How much HP and torque could a posi support and what kind of manual trans would you hook up behind that size engine? Thanks for any comments. Happy halloween.