Here's something from a Cougar site.
The engine was required to be stamped with a partial VIN beginning January 1st 1968. It was a part of the New Federal Motor Vehicle Safety Standard. The VIN on a 390 will either be on the back of the head or just under the back of the head on the block on the left hand or driver side. Since the heads are interchangeable, what was originally the back of the drivers side head would become the front of the passenger side if they were swapped around. If there is no VIN that you are looking at service replacement parts.