It's probably too late as you have already sold your truck it sounds but GMC pickups did come with a 228 engine but there is an exception to that rule. Canadian built GMC pickups had the wood bed bottom like a Chevy not the steel bottom of a GMC. They did have the GMC tailgate on them and a GMC grill but they put a Chevy 216 engine in them. Basically a true Chevy truck with a GMC tailgate and grill on it.
Also you are correct on the transmission. The three speeds are kind of rare because they were only made from 1941 to 1946 but the three-speed came out stock in all half-ton Chevrolet and GMC pickups unless they were ordered with a four-speed and from what I understand most of them wanted the four speeds.
