Don't do it. You won't be happy with the results. Just use R290 (Propane). I use Coleman. Compatible with R12 lube and it has better heat transfer. Of course you will need to do this yourself as it is officially illegal for techs to do this. It’s the refrigerant of choice in Europe and the Far East. Been running it for years. Black on black in Florida. Cheap and works great.

I was shocked when I first used my AC after getting my '87 last summer. Worked tons better than my newer fancy pants cars ever did. Go figure, 26 years old. Then again, I have a 30 year old fridge in the garage that's going strong. That R12 was becoming a real problem because when cars were scrapped, nobody cared what spewed into the air. Hoses were just cut to get the engines out. I should check with a HVAC tech to see what they do with an older commercial system.