The center cone was an early production option, not just on GTS's It was available on the 68 Darts that was produced in late 67 and was discontinued a few months later. I believe the mold got broke or something like that and they just discontinued it.
The second digit of your Vin will say what model/price class and the 5th digit will say what engine it came with. There should be a Vin decoder on this site to check it out.
Other than the fender tag, you could look for the factory bucket seat brackets welded to the floor, the factory dual exhaust hangers welded to the frame rails, the original holes for the GTS emblems on the fenders and deck lid. I'm not sure about the GTS but the GT's don't have a model emblem on the glove box door like the plain models do.