Sounds like you got an American GMC, as opposed to a Canadian GMC. Canadian GMC trucks used the Chevy 6, the GMC 6 is a superior engine to the chevy engine. If you check the horizontal pad behind the distributor, there should be a number stamped there. The first 3 numbers stamped will tell you the engine size. A 1950 American gmc would have had a 228 6 from the factory. However, a 248, 270, or 302 gmc 6 will bolt in where the original 228 was.
I'm crossing my fingers and hoping you have a 302 in there.
John
