what kind of posi rear end do i need for my 83 gt mustang or at least how do i find out?? kinda new at this so i need a little help

 

The rear end you "need" depends on what your car is throwing for horsepower, and what purpose your car has. If it is your stock daily driver, then you will have an entirely different set up than a drag racer. Is your car built? Is it stock? why are you repalcing the rear end? Please be more specific if you want any real help.