"3600" would designate a 3/4 ton truck. "3800" would be a 1 ton. Neither one would have 20" wheels- - - -My 1959 3800 had 18" 8 lug tube type wheels with an oddball removable bead ring that did not split- - -it had to be pried off using a couple of cutout areas of the bead ring. 3/4 tons usually had 16" or 17" 8 lug wheels. A truck with 20" wheels would be 1 1/2 ton or larger. It's possible someone has swapped a "3600" cab onto the heavier chassis, as all the cabs from 1/2 ton up are the same size- - -different fenders, hoods, and other sheet metal from a bigger truck will bolt onto a 3600 cab pretty easily, though.
Jerry
