Dual wheels were an option on 3/4 ton. I've seen 3/4 tons with duals all the way up to '72, though it is not very common.
As far as the frontend goes, you can remove the duallie adapter, which is hust bolted on, and run standard wheels. As far as a tire suggestion, that would depend on how wide of a tire you want on the rear. Then you get into load ranges, as many of the lower profile tires do not have the load range for that heavy of a truck. If you want to stay in something reasonable and easy to find, I would suggest a 245/75/16 E load range, very common size, roughly a 30" tall x 9" wide tire. A 265/75/16 E range may also work, not sure about front clearance. Those are roughly 31" tall x 10" wide. You'll need wheels with about 3.5-4" backspace to fit properly, pre-'87 4x4 should work fine. Another option would be to check into the later model Duallie wheels, run themw with the adapters and perhaps a 235/75/16, some also run 17s on the newer ones. I beleive they should clear the rear brakes on a 3600.
