Ok i have some questions to ask so here it is i have the 91 caprice 5.0 tbi and i want to put a stock 5.7 tbi motor in it and im looking for one with bigger hp and torque out of a truck or car what car or truck has the biggest hp and torque in it. Next if i switch the motor do i have to switch brains chips and or fuel pumps or can i us the stock 5.0 stuff. Next im wanting to make the car posi by switching the whole rear end out with something that will bolt in to can you tell me what cars i can get this stuff from.

Probably a vortec out of a 96-01 truck would be your best bet. Around 250 HP in stock form, but with an exhaust and intake, it would be closer to 275. You will need a new vortec intake as the ports and bolt pattern are different. You will also at the very least need to recalibrate the ECM. It would be preferable to just replace the ECM with something better. The two most popular choices are EBL and the later 93-95 TBI PCMs. Its also advisable to get a new fuel pump. The ACDelco TPI pump or Walbro 190LPH are two good choices. Better injectors and/or a larger TBI are also in order. 65PPH injectors would probably be the min. flowrate I would use.

Im not real familiar with which cars/trucks had similar rears to your caprice. I would assume that maybe a cop car would be the best place to look for a rear with limited slip. The other option may be to just swap out the carrier/gearset for something better.