Hi. I don't know if it is legal in the US or not, but here in Canada, there is an r-12, and r-134 compatible coolant called Duracool. I have been using it in many vehicles, including both my 80, and 83 Pontiac, for about the last 6 years. No issues. May be available in the US under a different name,

Easily available here, and the lines to install it. Best of luck. Jim.