VIN /TITLE matches drive-line but not body stampings. So yes VIN was swapped over to other body back in the 80's. Just wasn't something people thought much of back then.
What kind of document would that be? It would be like documenting that your drive train and interior weren't stolen...
but I understand the question...
I actually talked to Idaho DMV about all of this and wondered since it was a body swap should I actually apply for a state VIN?
They looked at me like I had a clown suit on, could have cared less....and really couldn't grasp the body stamping thing at all. Without a 17 digit VIN (like new cars) there isn't any database keeping track of our old cars.