What I would suggest is take them apart, clean them up good and inspect for bad rust pitting, cracks and make sure all the parts match. Also a good idea to not mix parts, it's a good practice to keep the rings with the dedicated wheel.
If all good, prime and paint, not too thick.
Install new tires tubes and flaps, you went all that way already, don't cheap out there. Anyway, usually come in a package deal when purchase new.
Use a good amount of tire lube, slightly inflate the tube and install both tube and flap inside the tire, then slide the tire over the lower part of the rim using lube on the flap, and make sure everything is at the right place, then install the top part. Rules of thumb, split 180° from the valve.
If you do not have a safety cage, you can inflate to about 15 psi, Mount them on the truck to inflate at a distance. For the front, wrap a chain loosely around, then inflate. Make sure all part are locking good. Go for a ride and recheck.
In my neck of the woods, 7.50-20 are impossible to find, if you have 6.00" or 6.50" wide wheels, go with 8.25-20. cheeper and readily available.
