I was just on the phone this week discussing this very matter with Classic Auto Air in Tampa, FL. They are experts on older factory A/C systems, and will replicate original parts, rebuild compressors, receiver/driers, etc. They did the main components for my 1955 Ford Selectaire. Right now they are building new factory bowden cables for my '62.
Classic Auto Air - Air Conditioning & Heating for 70’s & Older Cars & Trucks.
I asked about going to R134a. I was told, it does not let go of heat as easily as does R12. In order to compensate for this, it was suggested that I use a 2 row condenser out front so as to get all the latent heat out of the refrigerant, before it is returned to the dash unit. R12 systems, being more effective in this, only use a single row condenser out front. It's when you try to use R134a through this single row condenser, that most of the dissatisfaction with R134a results. I was told, (and already knew) that a new receiver drier is a must, as they do degrade with age.
Running 134a through a single row condenser will not produce a frosty cold A/C. Other than this concern, the Galaxies have a large enough system to produce a cold output at the dash A/C ducts with R134a.
I asked about using a Sanden compressor. It was highly recommended. The original compressor (Tecumseh, in my case) is a twin piston compressor that uses 14 horsepower to run it. It generates a load on the engine, heat, and can result in pinging with low grade gas if you are operating the car on a hot day. The Sanden compressor has 6 vanes and uses 2 horsepower. Your car will not even know that it's running.
I did not like the look of a polished metal Sanden under the hood what is basically a very stock car, as I am a stock guy. This weekend, I saw a Sanden painted satin gloss chassis black, and it looked good. It was there, but did not call out attention to itself. In my opinion, it is something I could live with. I never liked to open the hood on a restored car and see some new, modern part stuck on.
Classic Auto Air quoted me around $750 for everything from the firewall forward: new hoses, sight glass, new Sanden compressor and bracket to mount it in the existing A/C bracket on the engine, new receiver/drier, new 2 row condenser, hook ups and various fittings.
It looks like a new or rebuilt Tecumseh compressor is around $300, that damn clutch and pulley is about the same price. The clutch brush is $175 or so....then all the rest. It looks like around $1500 to go the original route. Then you have to add a fan clutch and shroud because of all the heat the thing generates.
The Sanden approach is looking better with each day. So it's just whether you want the factory original, or modern efficiency.
hope this helps,