If you're going to swap out the rear axle also, and want to keep the automatic, there are also other options.
Commandos from I believe 73 came with a TH400 and D20 setup. The stronger of the transmissions, but not a great low range T-case. The weak spot with this setup is the factory Trans to Tcase adapters. I found this setup locally for $350.
The full size trucks from around that time came with the same setup. I didn't know they also did until I saw one at the local pick and pull.
You could also go with an TF999 and D300 combo from a newer CJ. Better Tcase, but weaker transmission. How much weaker? IDK...but it ain't no TH400. I hear they are plenty strong though.
Some Dana 18's and 20's aren't interchangable. One being the mounting bore size. Two being the input gear tooth pitch and size.
Here's a chart that lists everything.
I'd guess I'm into my swap for about 8 bills. That's doing everything myself, and only putting in new gaskets and seals along the way.