I know nuttin about the 68 GTA, but how do you know it's a GTA without the original driver's door and data plate?
Never said it was a GTA. GT is for east coast and 68 and up. GTA was 67.
Without having original bill of sale, it's more difficult but not impossible. A few different items tell them apart. For starters, reinforced plates added in below the rear seats to accommodate dual exhaust hangers. Fog lights, grills, badges, hub caps, gas caps,etc.
I'm still trying to track original bill of sale. It's out there.