My 69 was a factory air car, but I replaced it with an aftermarket system from Classic Auto Air. Reason: It's running enough cam that I was concerned about making enough vacuum to operate all the factory valving.
When I bought mine, they didn't have a turnkey kit for 69 Pontiac A-bodies. (I think they do now) So, the folks there helped me sort of piece one together from components. That was the reason I chose them over Vintage Air. When I talked to Vintage, the conversation was very short, "No, we don't have anything for a 69 GTO." ....end of conversation. But the folks at Classic were willing to help me work something out.
I was able to adapt their system to fit my car AND work with my factory a/c controls. It took some doing, but I did it. Their a/c dash unit now is a lot nicer than what they had available when I did mine. The one now has 'electronic' controls that work off an ECM so it's much easier to calibrate them, especially when you're doing like I did and reusing your original dash controls. The one I have (all they had at the time) uses mechanical slide cables so it took me a few tries to get them attached with the right range of travel and so the positions on the various dash controls put the unit into the matching operational modes (heat, defrost, cool, temperature range, etc).
With all that, here are some things to consider: None of the aftermarket units I know of have any provision for cooling outside air. They're all in "recirculate" mode 100% of the time, meaning that they're continually cooling the air that is already inside the cabin. All of them require you to block off your outside air vents during installation. Also, at least in my case, the volume of air that my system moves is pretty wimpy even with the fan running at its highest speed. It's no where near the hurricane that my original factory system was able to blow, even though it's only feeding 3 outlets, all of them in the dash (driver, center, passenger). Everything else is blocked off and I did not install any lap vents at all. It does get the air reasonably cool, there's just not enough of it to keep up with the heat coming from the outside and through the firewall on a hot Texas day. Since I've got the engine out of my car right now, I'm strongly considering doing "something else" --- like attempting to resurrect my factory system and converting it to R134, going back to Classic to see if they have a better solution now, adding another layer of insulation to the firewall, or maybe seeing if there's some way I can get more air movement out of my present system.