[hatzie]

My cousin and I blew up a couple really decent used transmissions back in the early 80's because we didn't clean out the cooler loop in the radiators.

GM put the 65E in some of the Suburbans. Not sure about the pickups.

If you change the ring and pinion you will have to setup the axle... even if you swap the diff over with the gear attached. Used gears are "worn in" to the diff and axle housing they live in. The pinion will not run at exactly the same angle and depth as the gearsets' original home. I wouldn't swap the gears from one axle to the other in a truck you intend to keep. They tend to wear rapidly when moved to a new home.

If the matching front axle in the rust bucket is good I'd install it in the new truck along with the rear axle from the same truck. Then the gears will match.

You will need to program the ABS and PCM to match the installed axle ratio and tire size. ABS is a Tech2 operation. You'll need a tuner package to take care of the PCM.
You should clear any "learned" information from the PCM. Generally unhooking the battery for more than a few minutes will do this. It'll regenerate the tables from the way the engine and transmission etc work together after a couple hundred miles.