I recently installed a new belt on my compressor and had somebody put in 3 cans of R12 and 1 can R12 oil. Car cooled great for about a month. Then after 2 days of squeeling/grinding noise The compressor locked up and broke the belt. I am now ready to buy a new compressor, reciever/dryer and expansion valve, flush an clean system and start again. I was thinking about using the gm A6 compressor made of aluminium and us R134a. Any thoughts?

 

You had a major compressor failure and I would strongly recommend an inline filter. A few modest sized particles circulating will trash that pricy Alu A6 compressor quickly. All the components can be easily removed from the vehicle with exception of the evaporator. That can be cleaned in situ with a foaming flush. Everything else should be removed and cleaned or replaced.
The XJ condenser is a bit small for R134, it will cool to a point but on hot days when moving slow it will just give up. You can raise the point where it gives up by adding a larger pusher fan in front of the condenser.

 

there is a revised Harrison compressor sold by Retro Air, ( Porsche, Jaguar, Ferrari, MG, TR6, Alfa Romeo A/C Kits & Parts )