Just a guess to see if it helps. I myself am not a car buff. For cars and other over-the-road vehicles, (even collector cars I would think) I would expect there to be some type of transaction record at the state DMV. While VINs may not have been used back then, I would expect there to be some kind of serial number on the engine block maybe.
If anything, find and talk to another car collector to find out possible sources of information that may be available for you to establish an acquisition date if anything.
