Im looking to do the same thing for my 67 gs400. From what i have found so far ordering the vintage air 67 pontiac gto 961067 kit. But if your car was non a/c like mine im having trouble finding a/c compressor brackets and no one that i have found reproduces them. If you have found parts or any other info please share!
I have heard the Vintage air Pontiac system may be as good as any. It won't matter what you go with, there will be a bunch of fabricating. The simplest would be to go with a Vintage Air or Old Air underdash system. You will still need to build things like compressor brackets unless you use an old A-6 compressor.