rear end swap for different gear ratio

[Mattys1970C-10]

I just got a 1970 c-10 and it has a 3 speed th400. im pretty sure the rear end is stock and has 3.08's and no posi as it tries to rip the tire off the right wheel. id just buy a ring and pinion but i dont know how to install, so im trying to figure out an easy rear end swap from a different truck. suggestions on gear ratio for a mild build street truck rarely on the freeway, and what truck do i pull the rear end from?