For 67 and earlier, I'd recommend Vintage Air just because I know they make a kit for GTO's. (No one makes one for 68 and up, though)
However, I do also urge you to talk with them in detail about the system (since I'm personally unfamiliar with it) and find out if their system preserves your ability to vent in outside air and also choose between outside air and recirculate mode on cooling. The aftermarket system I have does NOT provide for any sort of outside air in either mode, in fact it doesn't have a "vent" mode at all. That may be a concern to you if you're considering replacing your factory air with an aftermarket system.
On the plus side, the new systems will be R134a instead of R12 for the original factory system, and R134a is much easier (and less expensive) to buy. Yes, there are "kits" to convert/recalibrate the factory R12 system over to R134a, but my research has led me to believe that doing that causes them to suffer a significant loss in cooling performance. Keep that in mind if you're considering that path.
It is possible to adapt an aftermarket system to work with the factory controls and vents on a factory air car, I did it on my 69, but it's a lot of work.