need a lesson in A/C

You cannot buy R12 anymore and that is what that system had or has in it, so it will have to be converted to R134a.

That will include and new drier at the very least. On systems that old, I always replaced the compressor and expansion valve, too. You will also need to replace every o ring in the system with R143a friendly o rings.

The system will need to be flushed thoroughly. I've always used a couple of cans of brake cleaner followed with compressed air until what comes out is clean and dry.

You will need to vacuum the system down good before you charge it. This gets all remaining moisture out.

The 134a charge is usually about 2/3 to 3/4 what the original R12 charge was. On a car with a pretty big passenger compartment like that, you can figure on somewhere "around" 36 ounces. It will end up being between two and three cans since they are 12 ounce cans now.

You cannot mix R12 and R143a together. You also cannot mix R12 oil and R134a together. This is why you must at the very least replace the drier, because the dried holds both moisture and a little oil.

This is also why I replace the compressor, especially on one that old. I have seen people remove the old compressor and "try" to drain the oil out, but it is nearly impossible and most times the old compressor will fail in a short time.