Phillip, I dont think there is much difference.
GMCs have a little elbow at the top where the fill cap screws on. Chevys didnt.
If there was a difference it would probably be the housing for the radiator as I know that is different between my 1947.1 GMC COE and my 1941 1 1/2 ton Chevy. The radiator in my COE currently does not have the elbow on top so Im sure its been replaced at some point, but without measuring the dimensions, they look very close to the same size.
Hope that helps. Do you have any pics of the truck? Id love to see some!
Jeff
