casting date needs to be before the build date of the car. So any car built in the 67 year would have a 67 casting date even a car built in January 68 would have a good chance of of a 67 casting date on many of the parts.
My father in law was the original owner of our car. The car was bought new on 5-31-68. The engine block, which is the original one with the car, was cast on 6-23-67.