Not too sure about the GMC truck hydramatics, but on Oldsmobiles and Pontiacs, the hydramatic transmission used from back in the 40s up through 1955 was what we called the "tilt case" hydramatic. In 1956 GM went to a different hydramatic in their cars. The tilt case unit had front and rear pumps and could be push started and also could be towed in neutral with the drive shaft in, as the rear pump circulated the tranny fluid. If the owner's manual for the 56 GMC says the driveshaft has to be removed for towing, it may be the later "slim jim" hydramatic. Probably just useless info, but I get a kick out of dragging some old car stuff out of my tired old brain.
Larry
