Switching to 134a can be done with a different compressor or with the stock one, either way. You won't see a huge performance difference change on it, but all things equal, newer compressors are better.
As for the LM7 compressor, that mounts down low on the passenger side and interferes with the crossmember. There may be mount combinations that will allow it to work, but I think you would have to switch to a car intake. You can cut and notch the crossmember if you are comfortable with that, or you can get a different set of brackets to put it on the main serpentine belt and use a Sanden style compressor. I believe there is one option that puts it in a similar location to the stock truck compressor but is a tiny little unit that might fit without modifications. Otherwise, a Sanden compressor is probably the way to go.
Condensers from maybe an Olds fit the stock radiator area and have the fittings on the correct side. Most of these are the modern parallel flow style, I believe, which will help. If you are swapping out your factory radiator, consider a stock Camaro radiator/fan/condenser combination. They have clips to hold each assembly together and is about the right size. Grab the hoses with it from the wrecking yard (cut on the rubber portions) and use the ends to make a new set of hoses (which will be needed for any major system change). Your factory style accumulator is fine, just make sure you install a new one after opening up the system.
I don't know of an upgraded evaporator or fan motor option option, but I'm all ears if someone else knows how to get a little more air through there...