I Have a 1976 Dodge powerwagon NW custom special W100. I am in process of a body Off build up. I am planning upgrading everthing. Starting with the axles. I plan on running a 39" or bigger tire but I am not sure on the axle combo that will best suit my rig and which donor trucks I should be looking at.

 

You need Dana 60s front and rear.
Find a front if you have cash or your lucky. Only use a Dodge front because even though a chevy will fit the spring pads are wider on the chev and you will get spring wander and twist. Bad.

A Dana 44 will handle 39"s if you drive nice. 36" max if you want to beat it.

In the rear you will find a Dana 60 in Dodge 3/4 tons and 1 tons. (2wd or 4wd)
Even a Dana 70 rear in some.

The 77 era Chev 14 bolt rear diff is also a direct swap in.
Found in 3/4 and 1 tons.
You want the FULL FLOATER. Stay away from the semi floater.

You want 4.56 gears with 36" rubber.
For 39" you going to want 4.88s or 5.14s.

I run 4.10s with 36" and wish i had lower.
Drive shaft mods are required but no big deal.