Real GTS or not.

I'd look for those dual exhaust hangers.

That'd be a good sign.

Pass side would be welded (single), my 66 Coronet 383-4 car has a bolted driver's side exhaust mount on the underside of the rear seat, but it is the same part as the welded one on the pass side. Having factory dual exhaust sure helps the case.

It's something you can do yourself, if you're waiting on paperwork based on SO numbers.