As with all anthropological claims, the bones are the proof. In this case, even the bones would not prove it. The 'bones' would be all bolt in so there will never be any conclusive evidence. At least not enough to change anyone's mind. Even if I could produce the car I saw with my own eyes and produced the original owner with his believable (or not)story. It would not be proof enough. You could find a picture from the late sixties with a Chevy engine in a Bird and it wouldn't be proof. TOHCan picked a task that was quite easy; Prove something to be false that was impossible to prove true in the first place. It will always take some amount of faith or unsubstantiated testimony to ascertain that any 1st gen Firebird had a Chevy engine first.