Jeff - I've done a little diving on the "ENG. NO. WE-1-9676" and can't find anything related to that designation in any of my regular resource documents. Considering that the truck was built in Tarrytown (confirmed by the "T" in the VIN), it would be most likely that the original 283 engine would have been built in the "Tonawanda" assembly plant as it's the closest proximity to the final Tarrytown assembly plant. A Tonawanda built engine would have a "T" as the first character in the stamped engine serial number, where the existing engine has an "F" indicating it was built at the Flint plant.
It could be (maybe) a slim possibility that the "F" engine is original to the truck when sold by the dealer, but at this point there's no real way to know for sure.
The only other resource I can think of that might shed a little more light on the listed "WE-1-9676" engine number might be a 1964 Chevrolet Truck Sales Data Book. Something I don't have the full book on, only access to the 2 change documents in the link I posted above. Those change documents don't address engine serial number codes.
On another thought, just a SWAG here, but I wonder about the engine ID listed in the "warranty book", if the "WE" stands for "Warranty Engine"??? In my thinking a warranty engine would have been restamped with that "WE-1-9676" on the engine serial number pad. That would make sense, but we have no way of knowing if that was the established procedure back then or not.
Sorry if this is too much rambling.
