Jon:
Anything's possible. These trucks are old and they've withstood aeons of abuse, which has caused many an engine swapout. If your truck has a history similar to mine, then it's entirely possible that the PO bought the cheapest running engine he could find and dropped it in.
My truck has had 5 engines in it. The engine that's currently in my truck was originally out of a '49 passenger car, via a '52 pickup that got t-boned. Grandpa got it for cheap and he and Uncle Skeeter did the swap in the driveway in the dead of winter. (History is fun!)
There should be a casting number down on the block or oil pan (can't remember how far down it is) and it'll be towards the radiator under the draft tube and the fuel pump. It's usually a six or seven digit number. There should also be a date code near the starter.
If the engine is older than 1946, then I don't have a serial number decoder chart that will work. I am fairly certain that all of the blocks came out of Flint or Tonawanda, but, since your engine might be Canadian, the numbers could be different.
Hopefully someone who in in the know will come along and enlighten us.
