If you go to the engine & driveline forum and search, you will find a lot of info. In a nutshell:
There is not a good solution. The best option is to find a rear end from a 64-72 3/4 ton with 4.10 gears. All you have to do then is swap the center section. And finding the right gears is getting tougher, no one is currently cutting any gears. Last ones were around $500 a set.
To replace the differential, the biggest problem is width, the later differentials 3.5" wider, and it's tough to get wheels that fit inside the fenders. You can swap a modern 14 bolt, with 3.73 or 3.42 gears, again, if you can get the right wheels.
I am running 4.10 gears with 7.50R16 tires (30" diameter). 60 mph is about 2600 rpm. I'm considering switching to a 700R4 automatic, which has a .73 ratio overdrive.
