All I can say is in the GMheritage website under 1963 truck, The actual scanned copy of the GM Chevrolet Truck spec states what I said in my post re: engine availability for K10. I would trust that more than your catalog book thing. I will concede that the engine you have could be factory, so as to be a good flexible Stovebolt member. Only 3 minor facts bring me there: 1.Being considerate to other very senior members, 2.Wikipedia and 3. the fact that your truck was built at about the end of 1962 production, right when the first 1963 production would have started to begin sales in Sept/Oct. that's when left overs would have been used, maybe.
Jim Carter and LongBox both say 261LB is for a BIG truck. Two good sources.
