If it's a real 69 Firebird 400, they made 11,522 total Firebird 400's (with an another breakdown between hardtop and convertible). They're not that easy to restore because they don't share too many parts with the 69 Camaro (other than the usual GM cross over parts, like suspension, frame, etc). Another thing that's VERY annoying is that the 1969 Firebird was unique in the Firebird line. It doesn't share too many parts with the 67/68 Firebird, and the 70-81 are a different bird altogether. Fortunately, the repro companies are really starting to wake up to the 69 Firebird, but it's a long, slow process, and lots of incorrect repro parts (or Camaro parts) have been introduced in the last 10 years or so to cash in on the Musclecar boom of the 1990's.
Values for the 1st gen Birds are now at what 68-70 GTO's are. The 67-69 Birds nearly doubled in price in the last 5 years. I have NO idea why.
A hardtop Firebird 400 around here just recently sold for $22,500. It was restored, but I noticed it had a lot of fiberglass repro parts, and a slightly modified engine.
A 67 Cutlass isn't that desirable, but the 442 line is unique and very desireable. The 67 Cutlass was not a Musclecar, the 442 was. Of course, if it's a convertible, the rules change. Ragtops haul big bucks, and sometimes people don't care whether it's a Cutlass, 442, or clone. They're all valuable.
The Mach 1 is good choice, lots of repro parts available. Not sure if the prices will continue to go up with the reintroduction of the new Mach 1, or the 2005 Mustang hitting the streets soon with the retro styling.
I would also vote for the 71 Z/28. They're just plain cool looking cars.
One thing nice about the Pontiacs, the build sheet is available through PHS. Most 67-69 Firebirds (6-cylinder, 326, or 350) have been cloned into 400 models, but the build sheet tells the story, and that's always available from an outside source (PHS).