Sounds like a good plan.

I have a Dana 70HD from a GM P-30 chassis, 10 lug, which is the very same at the wheels as the 80 you're looking at, the 22.5" wheels clear all the brakes stuff just fine and I expect the 20" wheels will as well. Remember the axles came with 19.5" wheels that clear.

Here's some reading about that conversion
http://www.stovebolt.com/techtips/bigbolts/p30swap/

And some reading about adapting disc brakes on the front if you like.
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=725572#Post725572

You ought to be able to find an axle for under $1,000. I forget what I paid for mine but nothing near that, and later got one for a friend for the price of two rotors.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-