A few years ago I installed a Sanden 508 and have been very happy. I bought it and a "York to Sanden" mounting adapter from Old Air Products in Texas. I believe it was Ft. Worth. The adapter bolts onto the existing bracket. The adapter and new compressor together weigh less than the Frigi-King (York) piston compressor and the rotary works so well that you can't tell when it kicks in. I also sent them my old drier and they cut it open replaced the media and sealed it back up. That way you can keep the period drier with inspection glass that you can't really find anymore.
For my application, I had a few cans of R-12 so I went with the model that is dual purpose. If I ever need to go to R-134, all I do is drain the compressor oil, put in the PAG-14 and use the 134. In the mean time, the R-12 gets so cold I could store meat in there.
At the same time I did that, I put in a new condensor that had more, smaller diameter tubes in it. I'm sure that helped with the cooling efficiency. It came from a source in California and has the same volume as the original condensor. It's important for the condensor side and the evaporator side to have the same capacity for good balance.
Good luck. I know you'll be happy with the results.