VINs became standardized in the 1980s. Our old trucks do not have a VIN. They have a body serial number and an engine serial number. Some states used one, other states used the other for registration purposes. I believe California used the engine number.
If there is no door jamb tag your body number is lost forever on a '51 Chevy truck.
As for the GM Heritage site policy, you'd be better off asking them. If you need a body tag number for that purpose, make one up based on an actual '51 number. The info you receive from them is general, not for a specific vehicle.
