In the photo, the area highlighted in green. Between the top of the rad support and front fascia I guess you'd call it. What are these missing flat pieces called, and where they present in all 1967 Firebirds? Are they hard to find? Seems like they would be a good thing to have in place to direct as much air as possible to the radiator, also just has a cleaner look than a big hole....

Radiator core support filler panels. They came on 400 cars and cars with AC. Ames sell repops, page 236 in catalog. They direct more cool air into the radiator keeping the hot air in the engine compartment from getting pulled back into the radiator by the fan

Thank you sir, appreciate the help and reference in Ames.

In cars you see that have custom engine bay panels, are these always one-off custom made pieces or is there someone out there making these panels? Talking like a one piece that covers the whole front core support area or inner fender cover panels? I probably have the terminology wrong.

There could be but the factory pieces were 2 piece withe an”X” stamped in them. You might occasionally hear them called X panels.