The first thing to check is what code, date, and if the motor is a service replacement or not. Does the serial number on the block match the car? I have heard the dealer stamped the service replacement blocks with the cars #. The 2nd thing I would do regardless is to send away for the original paperwork from these guys http://www.phs-online.com/ to find what the car was originally sent to the dealer with and who the original selling dealer was. There were to main dealers that are known for 428 swaps , Myrtle motors and Royal Pontiac but there were others. If the engine checks out with a full size code and a partial serial from one I think the chances are slim to none but by all means I am no expert on these special cars and am only regurgitating what I have read over the years sprinkled with memory loss.