How do the rivets holding the VIN plate on look from underneath the dash? That's usually a big giveaway if it's been tampered with.
I once briefly owned an El Camino that had been "assembled" with parts from a bunch of different donors. Imagine my delight when I discovered that it had horrible grinder marks underneath the dash where the VIN plate attached, and none of the glass date codes were a match to the VIN. Traded it to a guy that wanted it to build a Cutlass Camino...fully disclosed it to him, he cut the firewall out of it and used the cutlass cowl on it.