So I'm thinking about buying a 1981 pickup that has a 20R in it. It runs but the first and second gears do not engage so I was wondering if rather than rebuilding the transmission if I could just swap the g58 out of a newer pickup in to it? Thanks for any help guys

 

You are limited by the length differences in different transmissions, many will bolt up to the engine, but your crossmember and drivelines wont match up, get an L series transmission if you want a direct bolt in replacement.