[Short 3/4]

Glad to see you are trying to keep that old survivor original. That old obsolete stuff is what makes your truck unique, not just another hacked up "improved " one.
If the shaft you have is 1/2 ton you may be able to get a conversion type u- joint to make it fit. Not something I would want for permanent, but good enough for a test. Like I said, I was convinced I had a transmission problem, but turned out to be x-fer ( of course on our old junk, it could be both)
For what it's worth, I was very happy with the results of a swap to 4.10 gears. Truck now cruises just fine at 60mph, which is as fast as I really want to go in one of these old things (if I'm in that much of a hurry, I have faster cars in the fleet)
So how were you able to make those 15" rims fit over your drums? I've seen some 3/4 ton C trucks with 15's but they have smaller brakes than the K trucks.
Glad you are making progress on the project, have fun with it
Tom