The more I learn about automotive air conditioning, the more I realize that an R-12-based system is the way to go.
I still have three vehicles (including a motorhome) that have R-12 Freon systems, and although auto repair shops pretty much all recommend conversion to R134, R-134 just doesn't cool as well as R-12 Freon, even in an air conditioner that was designed for it. I used to have a 1979 Jeep Wagoneer with an R-12 system that would literally blow ice crystals on a hot day. R-134 air conditioners don't do that.
And then there is the huge expense. The only way to properly convert to R-134 is to replace everything in the system; and anecdotal evidence suggests that if you do convert to R-134a refrigerant, you will replace everything: because anything you don't replace will not be long for this world, and when it goes it will probably take everything else with it.
Fortunately, there are a couple of refrigerants available that were designed specifically to be a direct replacement for R-12 Freon. One of these is Freeze-12. According to the manufacturer's website, all you have to do to use Freeze-12 is evacuate the old Freon and recharge with Freeze-12, because it is compatible with the lubricating oil in the R-12 system. You can get everything you need to recharge your R-12 air conditioner with Freeze-12 right here:
There are other R-12 replacements out there as well, but Freeze-12 is the most well-known and accepted.
But there is a better way, too. You can build your own DIY air conditioner system that will cause icicles to form on your nose, and if done properly will never need to be recharged. A lifetime system. And it will not cost any more than a proper R-134a conversion. Here's what you do:
First, get rid of the old, inefficient compressor and replace it with a newer, efficient design with better seals so you don't lose the refrigerant charge. You may as well get a big one, because it won't cost much if any more than a small one. Something like this Sanden compressor, which was factory-installed on Freightliner trucks:
Once you have the compressor, you will have to fabricate a mount for your engine. If you don't have the setup to do that, just look for a local shadetree mechanic to help you. It is really not difficult.
First though, take all the hoses off your existing system and take them, along with your new compressor, to an air conditioning service shop. What you want them to do is make new hoses using barrier hose and crimped connectors, to fit your existing system. Except, of course, have them install the proper compressor fittings to fit the new compressor, as they will likely be different from your original compressor.
You can use your existing evaporator and condenser (assuming they don't leak) or install new, larger ones if you have the space and want an even colder system.
While you have it all apart, replace the drier. You don't want the old one ruining all your work. Then just install the new lines and compressor, evacuate the system for several hours, put in a new oil charge, and lastly, put in either Freeze-12 or, if you can find any, R-406a.