As mentioned,there are no VIN numbers on 68 engines and transmissions.
However,you can verify if the drivetrain is original to the car by checking the 14 digit serial numbers stamped on the cylinder block pan rail at the left rear corner below starter opening if it is a B/RB engine.
Typically should be within three months or less of the shipping order number.
Trans has a PK number on the left side oil pan rail.
If you have access to a 68 FSM,it can help decifer the numbers.
Hope this helps