Though I'm happy to volunteer to help you out, this one is a little outside of my area of expertise. I am still learning the ins and outs of the Firebirds with respect to cowl tag decoding. I would guess that the cowl tag is really there, just hidden under a coat of paint or some grime as mine turned out to be.
As for the authenticity - there's no substitute for the PHS documents. I would advise you to order them if you are seriously considering the car for its collector value. IF you are just loking to make it into your own personal project car, then perhaps the price vs. condition would be your most important factor.
