73 dart, where to start

Futzy, I'm not sure what it would take to piecemeal a full blown aftermarket system together since apparently there are no bolt-in kits for your application. I do know that there are self-contained AC systems that are mounted under the dash. Granted, I'm not sure if it will work for your needs. One thing you may want to consider is finding a similar A-body to yours where someone is removing the entire system (for whatever their reason is) and grab every bit of it - refurbishing those pieces that may need attention. Best case scenario is to find someone local and actually help them remove it so you can see how it goes back together and again, to make sure you get every part you need. Who knows, they may just give it to you for the labor. GL.