How do I charge the A/C on my 1980 mercury zephyr?

Me and my friend own a 1980 mercury zephyr which is basically a ford fairmont that we want to charge the A/C on but nothing sold now seems to fit the filling hole. We have tried looking around online and it seems that theirs no info, if you know anything mind helping out?

Any Ford Fox platform vehicle, such as the same year Mustang or Capri.

My guess is that you need to buy a kit to convert to R134 refrigerant. In 1980 cars used R12 which is no longer available. R12 has different fittings than 134.

Look for an R12 to R134a conversion kit.

To change your ac to one that can use R-134a; every drop of oil in the system has to be flushed out, from the compressor, lines, evaporator & condenser, and replaced with a suitable "PAG" oil. This is no easy task. Then the expansion valve or orifice tube has to be changed. Why? The two different refrigerants use different and non-compatible oils, and also the less efficient R-134a will use a different expansion valve because of its different properties.
An R-12 system conversion to R-134a will never be as cold inside the car as with the original stuff.
Consider going with 'Enviro-safe' refrigerant. It's available online and in some stores. It's compatible with the oil in your system now, it's a bit colder than R-12 so your car will cool down nice, and you don't need any special license or permit to buy it. The one drawback is that it is flammable when exposed to an open flame, where R-12 & R-134a are not.
Besides using R-12 in your system, these are the two choices. Do some research and find the best one.

That's because you need R12, which you won't find. Have a shop convert it to R134A.

The reason none of the current recharge kits fit is because they're all for R134a systems and yours is an R12a system, which is obsolete since R12a can't be sold anymore. You will need to install an R134a kit on your low side valve before and R134a can be charged in. You need the system evacuated by an AC tech and new lubricant installed that works with R134a. R134a is not as efficient as R12a was, so after it's recharged it will work, but not as well as it originally did. There are lots of youtubes on this. Lookit

Do a search for adapters for when changing from R12 to 134a refrigerant. Auto parts stores can order them if they don't stock them. Ask the counter person for the adapter kit. They are easy to just screw on.
When buying a couple cans of 134a get just one that has the sealer in it and the other can plain 134a. That way if there's a small slow leak it will seal it.