The things 1bad Indian mentioned are a good start But I myself would be checking the hidden vins. under the cowl cover and the blower motor cover to see if they match the tag on the dash. Also look closely at the rivets on the vin a cowl tags the vin uses a rosette style rivet while the cowl tag uses a round style with a big hole that is filled with seam sealer. The next step would be to contact the owner of phs as he has seen everything and would ask the appropriate questions to help you get to the bottom of this. Knowing the original owner and the history of the car is a big plus for you. I Heard of a GTO post car with an improper number showing it to be a non post car and and after discussing it with phs the owner discovered it was one of a few factory verified numbering mistakes so instead of it being modified car worth less it became a bragging point. I hope you figure it out, let us know.