Here are 1954 GMC heaters (four styles). At least one of those heaters are slightly different from the earlier GMC heaters. This price pad shows that there were 6 GMC heaters in 1951 (1952 would be similar/same-as 1951). Four of the GMC heaters are different from the two common GM Chevrolet truck heaters.

GM heater motors would have been the same for similar style and voltage GMC/Chevrolet-truck heaters.