that's a good truck, i have a few very similar, 9430s and 1434s
the block 3695451 is a 216 short side cover babbitt engine, could be the original one, the plate number should be found stamped on the flat machine surface next to the distributor if it is the original engine.
yes that's a chevy engine, your GMC is a Chevy for all intents and purposes.
you will find that things on your truck will show "made in Canada" or "gm Canada" etc... and always consider that the USA truck sometimes had tiny differences so when you order a part for your Canada truck, keep in mind that even if you get the "wrong" part it might be the right part for the USA
check ebay canada, the manuals for the Canadian trucks show up there once in a while