Well it could have been done by engine number.
with what you have you aren't going to get a normal
vin number that has meaning for a 50 because you have no idea
what the number was for the truck you now have.
the only way to get a vin that makes has meaning is to go the
wall art route essentially using the vin from a hulk that is beyond repair a parts truck.
kind of the inverse of doing a cab swap
it's done all the time with old stuff with no criminal intent