The only thing left to decide is whether or not to retrofit your car to take the new R-134a gas - which is significantly less expensive and can be purchased by anyone (no licenses needed!). The "experts" opinion's of the retrofit procedure vary widely - some say you need a larger condenser, compressor and evaporator(s) as well as the new barrier AC hose. Other's say you only need to replace the receiver/dryer, flush the system, install R-134a compatible oil (Ester oil) and change the service fittings. The deciding factor for me was that the entire system had to be flushed to get rid of all the old mineral oil -and with two evaporators, that is not a pleasant prospect! Apparently, the R-134a gas will react with the mineral oil to form sludge which will clog up the system, eventually leading to compressor failure. 
I decided (eventually) to keep using Freon (R12) gas for now, at least until more information is available on the new R-134a gas or even one of the newer "drop-in" Freon replacements, like FRIGC-FR12. The latest information on retrofit options (well, at least the US governments opinion...) can be found at the US EPA's web site 

This is the complete 'kit' you will need to install this compressor. I bought the components from Alfa Parts Exchange in California. The price was quite reasonable. 
Note that the fittings to connect the hoses to the air conditioner have been removed from the old York compressor and installed on the new Sanden compressor. These also contain the service ports needed to recharge the system. I always kept the AC system capped off (in this case with rubber plugs) since any dirt introduced to the system is potentially fatal to the compressor! 

You will need to drain all the old mineral oil from the compressor and refill it the correct amount of oil. In my case, this was 8 ounces. Always use new o-rings - for the GTV6, there are 3 different sizes of o-rings needed. 
I removed the headlights, grill and air-dam in order to clean the condenser and replace both o-rings on the condenser to hose fittings. 

If you want to buy a new compressor, then try to find a Sanden SD-508 model number 9529. I could not find an exact replacement receiver/dryer - so used a more commonly available replacement (any one of these): 
SCS/Frigette # 207-237, Everco #A9100, Murray #208318, Delco #15-195, 4 Seas #34259.  
Note that this only has one port available for the compressor cut-out switches - so replace both switches with a hi/low type switch that will switch the compressor off if the pressure either exceeds a given level or falls below it (low refrigerant charge). Note also that this newer receiver/dryer is quite a lot taller than the older unit - but it will fit well within the existing bracket. 
You will also need a new Milano AC belt since the old GTV6 one is a little long for the newer compressor.  

This car is a 1985 model and is fitted with the Tropic-Air AC system. Basically, this means it has an added evaporator in the passenger compartment - and hence the second suction pipe that attaches to the compressor. There is still enough space to remove the oil filter. The hoses do pass rather close to the manifolds - nothing besides major surgery and new hoses will cure this problem. I found though, that there is at least 2.5 cm (one inch) clearance so do not expect any problems. 

The installation itself was quite straightforward. Remove the large (17mm I think) bolt from the block and replace it with the threaded stud. I decided to completely assemble the bracket and compressor before fitting the entire assembly into the engine compartment (this included the idler pulley). Slip the assembly over the threaded stud in front, then install the 2 bolts from beneath the car. I had a helper assist to line up the holes here. 

Once the AC compressor, receiver dryer and new o-rings were installed, I took the car to my local AC mechanic to have the system charged. Unfortunately, he was unable to find any reference which stated the volume of gas that the Tropic Air system takes. We decided that the average car with a Sanden compressor and one evaporator takes about 32 ounces - and adding an extra 6 ounces for the second evaporator worked well. Just as the last few ounces were being added, the sight glass on the receiver dryer went clear - indicating full charge. 

The air conditioner is more efficient now, and certainly smoother. I tested it out on one of our S. California 110 degree days and it worked well - and it really looks a lot cooler ... ;-) than the huge old York compressor!