The 1971 duster in Missouri, I'm going to contact the owner. I can get ac added for a couple of grand and since it's a 3 speed manual gives me a good reason to swap it for a 4 speed.
Adding AC to a non-AC car is gonna be a "couple grand" just in parts, plus the labor to install, and it'll take more than a day. So, adding the AC is gonna be a few grand, at least. Just check ClassicAutoAir or VintageAir, you're not getting a complete system for under $2k. That Duster appears to have a 19" radiator as well, so, that might not work out with the demands of an AC system. Not 100% on that but it looks smaller than the 22" radiators in both of my '71's. Depending on the system used you'll also need pulleys, you have the non AC water pump, and you've got a manual steering car which requires some extra hoops to jump through because most AC cars had power steering, so the tensioner needed with manual steering up isn't common.
Heck, you might need to upgrade the alternator too, since that will be the little one on there with the complete lack of accessories on that Duster!