Damn. Sorry. I had no idea they were unavailable. Now I know, and I'll stop sending people their way. ..shame. They should be able to load a sheet of 3M Vinyl into their plotter/cutter and "print" new ones on the spot. Maybe they are no longer authorized to reproduce them?
I might suggest contacting places that restore these cars, maybe a little "brotherhood" will go a long way. There's a Classic Car Museum/Showroom with locations in a couple states called Hanksters. The guys in the Daytona Beach location were really friendly, knowledgeable, and have a passion for these Classics that borders near insanity. I do not know who "restores" their cars, but they restore or "clone" them to Showroom condition including all the various color paint marks, etc. They or their guy(s) are very meticulous. It wouldn't be a stretch to call them Concourse level restorations.
Can't say I've seen a 1st Gen Firebird on their floor, but the work they do (or have done for them) on first Gen Camaros and other makes/models is certainly well above average.
Almost bought a fresh Yenko clone from them 5 years back. Even though it was a clone, it was correct down to every last detail. Even the L72/M21. Only $45k for a fresh restoration. Compared to a genuine Yenko which was going for over a quarter-million, it was hard to walk away. I could have afforded the financing terms without problem, I just didn't want the financial commitment.
Anyway, maybe they could put you in contact with someone who can advise you, or at the very least direct you to where you need to be?
That's all I got. If I owned one, I'd happily go out and measure for you.
Good Luck! ...again, sorry Phoenix and Classic Industries shot you down. Did you try Firebird Central too?