That rear axle is from a 1955-64 passenger car. If the housing is set up for leaf springs, it would be from a 1955-57 year vehicle. 58-64 cars ran coil springs and a different set of axle tubes on a center section with the same bolt pattern for the carrier housing to bolt into. The most recently manufactured versions of that rear end will be 60+ years old, and aftermarket gears and rebuilding parts are anywhere from scarce to nonexistent. It will also have a 5 lug wheel bolt pattern, different from the 6 lug hubs on the front end.
Ditto on the 3 speed overdrive transmission. The SM-319's with the Borg-Warner overdrive are out there somewhere, but you'll have a lot of difficulty finding one that doesn't need a rebuild, or at least freshening up with new bearings, gaskets, etc. The electrical controls, particularly the relay that engages the solenoid, are really scarce. If you're not into doing smoky burnouts with wide cheater slicks on the truck, a T-5 overdrive 5 speed transmission is 50 years newer and it will fit the same bellhousing bolt pattern as the original 3 speed.
My father had a favorite saying- - - -"Automatic transmissions are smarter than all drivers, and most mechanics!" Since he was the first man to work on automatics in Nashville Tennessee in the late 1940's, I think he was qualified to make that assertion.
Since you're not planning to do a numbers-correct restoration anyway, I'd suggest going with a small block V8 of some size, and a 4L60 automatic overdrive transmission. There are programmable stand-alone electronic controllers available that eliminate the need to install the full computer control system it needs to control shift points. RUN- - - -do not walk away from the 4L60's parent trans, the cable controlled 700R4! the Rear end- - - -there are a few versions of a 6 lug axle for S-10 4WD that has a 6 lug bolt pattern and a suitable gear ratio. If you insist on having a standard trans to play with, the T-5 and S-10 rear end play nicely together.
Just one man's opinion- - - -probably worth exactly what it costs you to read it- - - -but it's based on 60+ years of tinkering with these old rust buckets. Good luck!
Jerry
