I put a Classic Auto Air unit in my 69 Barracuda about 20 years ago. The instructions were not very good at the time, but the guys on their tech line were excellent. The instructions are better now. It still blows ice cold.
I am looking for the same thing for my 74 Duster 360. I just want an under dash unit since I rebuilt my heating system when I finished the restomod in November.
Classic Auto Air has an under dash unit and can put together all of the underhood components for about $2K. Looking at Vintage and Old Air systems as well. It's too bad no one (yet) makes an A body kit.
Let me know what you choose and how the install goes.