[EC900]

Congratulations on the acquisition. Post a photo, love to see original examples.

There should be a sticker somewhere on the rear lid that will determine the Freon refrigerant spec or if there was any conversion probably in last 30 yrs since R12 was outlawed, or some literature in the owner’s manual was the original system had. Comparing to a modern car, the R12 ports are smaller than R134a.
I had a new 91 Honda w/R12 that had to be converted in 1996 (large evap leak) because of the industry transition to R134a was in affect and R12 was getting scarce even back then.

I too agree with colingreene, take it to an AC shop and at very least first get it pressurized. Good chance there’s a leak, dry rot seals and the aluminum fittings have a tendency to corrode over years….decades.
Dealer installed or factory?
There’s a bit of difference between the two. Dealer installs were rough installs, chopped a hole in the passenger side foot well.
These AC units were never a superior system whether factory or dealer installed VPC units, going through the expense of any diagnostics and repairs, satisfactory results are going to be iffy especially on the sunniest of hot summer days.

Another thing to consider is the engine and exhaust - whether or not you have a 5 blade fan and thermal reactors which create more engine heat (for emissions burn off) then adding the ac compressor to that load… in a traffic jam or city stop n’ go, might be a long term detriment.

You going to be better off budgeting for a modern system.