1968 Dart GTS 340
Doesnt the VIN number establish that it is a GTS?
BTW, nice car. If I had the money I'd be all over it.
No, the VIN alone does not establish whether the car is indeed a factory GTS.
It is a well known step that some people take, to re-body a car, by transferring the dash with the VIN plate, the doors with the manufacturer tag, and the radiator support, from a real factory high performance Mopar car to a lesser car that may have a decent body/structure. This happens often with the rare higher end Mopars like 'Cudas, A12s and Hemi cars, and if you were to compare the number of these cars that the factory actually built, versus the number of cars out there for sale, claiming to be the "real" deal, you might find that the number of cars claimed to be "real" versus the total number produced by the factory could be way out of wack.
The car should be stamped in numerous places, both on the drivetrain and on the body / radiator support, that can validate the car.
The best source would be a combination of the VIN stamps on the vehicle, and ownership documentation of the car's history, from the initial purchase date.