Roadrunner or Not

IF it were 69 or newer the vin or partial vin would be in the hidden locations. This is a 68 there are no hidden vin's. The numbers on the core support and trunk lip correspond to the fender tag and not the title or vin. If no vin tag, fender tag and or build sheet... there is nothing.
To take this to DMV or police to check or reissue a vin there is a chance they will seize your vehicle.

That's the crux of the problem. Assuming that Diana's Dart is an honest person, her and her husband are in a precarious position just the same. I would guess they paid a good amount of money for the car. One of the last things they want to happen is to have it taken from them.

I would suggest that they do their best to make the person that sold it to them provide documented proof of the car's history. I wouldn't contact legal authorities yet. The risk of ending up with nothing except a large hole in your pocketbook is too great. I would take steps to protect myself while being careful.
Hopefully the seller isn't a dangerous person and they shouldn't put themselves at risk.

It may sound as though this is over-reacting to a situation that may end up being nothing more than a VIN tag lost during a restoration. But when a large sum of money is involved caution should be exercised if you don't know the other party.