The rear axle in my '53 GMC 1/2 ton is a stock housing, brakes, axles, etc. I replaced the original torque tube differential with a '55 - '62 3. 90 differential .
You'll want to start by setting your pinion angle and welding the spring perch pivots solid. Then it's as simple as removing the rear cover, disconnecting the axles and moving them out a bit, unbolting and removing the torque tube differential, sliding the new differential into the housing (with a new gasket of course) and bolting it in place, sliding the axles back into place and connecting them, replacing the rear cover, and filling with gear lube.
Now of course you have to find a driveshaft or you're not going anywhere.
