There were several companies that made those type conversions, also ther was a dual wheel factory option on 3/4 ton too, but that would have had standard dually wheels and no adapter for the rear.
In a 15", the closest would probably be a 215/75/15 or 225/75/15, but they are not being made in an 8 ply equivelant, 6 ply is the closest that is currently made. If you don't mind loosing a little originality, I'd suggest going to a 1 peice 16", preferrably for an early '80s 4x4, and a 215/85/16 E load range (10 ply).
