You might see if you can retrieve some past threads regarding this. From what I've gathered R12 is no longer being manufactured tho there are tons left. As you know it (the single cans) is available on eBay/CL tho it is pricey. To changeover I think you need the new type "barrier" hoses, the reciever/drier needs to be changed to the new type, the valve at the firewall needs to be changed, the oil in the compressor needs to be changed to the new type, new type of O rings in the hoses/fittings then vac pump the system empty which is std procedure then add the new stuff & I think you go by weight & use less of the new type of freon. Some say it dont cool as well as R12 & others are happy with it. dont remember if a larger evaporator (next to rad) is needed (or recommended). I'd wanna check the system for leaks in the process so I dont have to mess with it again. Might be more to it than this but here's a BTT for ya. EDIT A '66 "300" would have a monstrous condensor so you're good there