Having done some research on the topic, the problem with converting from R12 to R134a doesn't have anything to do with the compressor. As long as you start with a clean and dry system, the compressor doesn't know or care what kind of gas it's squeezing. The problem is with the part that allows the compressed gas (liguid) to evaporate into a gass and get cold. This was usually something called a POA valve on the factory systems. That's where the calibration is that "cares" about what kind of refrigerant it's dealing with. Problem is, you can't get them anymore - for R12 or R134a - or if you do find one it's hideously expensive and will need to be recalibrated for R134a. There are things called "POA elimination kits" that replace that component with the more modern orifice tube setup, but I've heard that they really don't work very well (despite being promoted by lots of sites that sell them).
If you're going to run that system, it's best to just go ahead and run it with R12 the way it was originally designed - assuming your POA valve and all the other components that "see" gas are in good working order, don't leak, haven't rusted, etc. It's still possible to purchase R12 and deal with it yourself, but you do have to be "licensed" to do so. When I was looking into doing that myself I found several internet sites where one could take the necessary training and get certified for the license. It didn't look too bad to do.
I still had 100% of all my original factory air components (and still do), but when I started looking at what it was going to cost for a new compressor (mine's bad), all the other components that needed replacing (including the POA valve if I could find one) it turned out it was going to be more expensive than just replacing the whole system --- and I'd still have the vacuum problem due to the cam.
Your situation might be different though and I don't want to talk you into something that's not a good idea for you - I'm just encouraging you to do your homework and figure out what it's really going to cost you to get the factory system going.
I just got done a few minutes ago evacuating and charging my new Classic Auto Air system for the first time, and it's working great. I've got a "cool" car now
I went through quite a lot to adapt the Chevelle kit (no one makes a turn key kit yet for the 69 GTO - 67 and earlier yes, but not the 69) to my car and retain as much as possible all the original factory controls - but I did it. Holler if I can help in some way.