Welcome to the "Bolt". I guess that you don't have the original rear end? The 3/4 T would have come standard with a 4.57 ratio. There are threads about this but basically the 3/4 T automatics with coil springs from 67 to 72 will yield a 4.10 pumpkin which is a direct bolt in for the original setup. You can read more about this on the Jim Carter site.
Good luck with your truck. We all really like photos around here!
