Sorry we are really not much help jkane. This topic has been talked to "death" almost - here and on lots of other old car/truck boards - but there is still lots of misinformation. As others have stated some states used the original engine # as the VIN on the title. This was common until the late 50's(?) But others did use the body serial number and I think today all jurisdictions would consider the body number as the VIN. But I do not think you would have any trouble with the title of an old truck that had the engine # as the VIN and it still had the original motor or if you could demonstrate that the number was the original engine number; the correct numbers and letters in the right places. It probably depends on who you talk to at the DMV.
Somebody should write a book and include the rules in each state! As for the replacement tags - they are probably illegal in every state, my guess. Lots of people must use them though - I see "historical documents" for sale all the time.
Good luck.

Professional Novice