Hy Iron Clad, thanks for the pic's, that is a corporation (GM) single speed rear axle. I think those axles were 15,000 lb. max., most of the ratios I have found in my parts book for that axle are in the low to mid 6s which should give a decent speed. What size tires are currently on the rear of the truck? That transmission casting number is very close to a case p/n for a Muncie four speed.
