Aftermarket A/C for '66, /6 Dart

This is a bigger, more expensive job than you might realise. There is no model-specific kit for that car, but you can put together a system that'll work as well as the system the Dodge dealer would've installed if you'd brought the car to them after purchase. It'll involve an under-dash evaporator unit and a bunch of carefully-chosen other parts (parallel-flow condenser, etc). There's a mount/drive kit to put a Sanden compressor on a Slant-6, but it's not very thoughtfully designed and tends not to work very well without modifications. You'll need to upgrade the radiator and fan; see here and read all five parts of this article. You'll also need to apply carefully-chosen window film, extensively insulate the car body, and find and plug all the existing holes (there are many) if you want the A/C to do more for you than sit there making noise and drinking gasoline. Read this and here and links from those.