There are just so many options and ways you could go with the rear axle. Hard to list and evaluate them all at once. This is a short version, not attempting to answer all the questions and considerations that each may bring.
If you want an original bolt pattern for these trucks yet hard to find 22.5" wheels you're looking for a Dana 70HD or Dana 80 from a GM P30, or 3500HD, 10 lugs on 7.25" bolt circle. These come with disc brakes only, and no parking brake.
As used in this project
Picture Album I'm unaware of anything better/newer for your actual original bolt pattern if it is 5 lug Motor wheel?
I have seen 6 lug Motor Wheel 22.5" wheels, and they might have made 5 lug, either would be exceedingly rare
Picture A more common bolt patern yet still reasonably old, would look almost appropriate on your model truck is 6 lug Budd stud centered. 22.5" wheels are pretty easy to find. Axle choices are plentiful. The smaller ones would be a few special/odd Dana 70HD and 80 again. Then larger stuff as found under school busses and other medium duty trucks. most of these older choices will have drum brakes and some later models with disc.
This shows a Dana 80 with this 6 lug Budd bolt pattern.
Picture Album Next if you don't care what wheels the most common these days will be hub piloted 8 or 10 lug as found on most every medium and heavy duty truck in the past 10-20 years (more or less). wheels are extremely easy to find, steel and aluminum, 19.5", 22.5" and larger.
I used a Rockwell Meritor RS-15-20 15,000 lb axle with 8 large lug hub centered pattern in this project
Picture Album Keep in mind if you put disc brakes in the rear you relaly should first have them up front.
If you use drum brakes in the rear and want a newer axle might have to search a little to find hydraulic drums instead of air drum brakes or hydraulic disc.
Planning the truck project you're contemplating is a lot of work and considerations, and worth doing before buying parts.
As to the cost, it'll be a lot if you want it done nice and tidy and safe, and even if you do all the labor. It's all the little stuff that adds up to make a very expensive project. The cost of the engine will be a small part of the whole project. Don't pick an engine that is harder to fit and deal with just because the right choice seemed to be "expensive" at the time, you can make up that difference in "savings" with time and aggravation pretty quick.
Grigg
