Hy Beaster, it looks like you have deciphered your serial number correctly, that engine serial number is correct for a Canadian GMC 248 engine, but your pictures clearly show a newer Chevrolet engine in the truck, so the engine has been replaced at some point. You will probably find some kind of adapter plate at the front of the engine which bridges the gap between the GMC engine mount on the chassis and the shorter Chevrolet engine, hope that helps.
