1984 Chevy Monte Carlo convert r-12 system to 134A

I recently accuried this car but the A/C is not working. The pressure switch will not let the compressor magnetic coil enguage due to low or no gas in the system. I was told that the expansion valve must be changed to accomodate the new 134A gas as the pressures are diffrent and R-12 and is very expensive. I can't find find the expansion valve. This car comes with a radial type compressor diffrent to the older model which is also round but almost a foot long. I noticed by the evaporator compartment their is a large nut that couples to another pipe with hose that goes to the front radiator which the pipe had black dirt on it giving a sign of oil leaking out from it; hopefully this is where the leak is. I spoke with a friend who does commercial/residentials a/c and told me I may need to install a new compressor that works with the new 134A gas. I am not a professional mechanic but with the right instructions and component I am sure I can task this job. Now if a new compressor is needed how can I get the proper brackets for it to fit this GM 305 V8 engine. Is there a simple solution to this problem and what do I really need to change out for this coming summer in Naples florida.