I'm sorry, but I can't keep from offering my unsolicited $0.02 here.
First of all I would highly recommend against a rebuilt R4 compressor. I think Bill has obviously had better luck with them than I have. The R4 does not seem to rebuild well. If it fails it will leave a real mess in the system that will have to be thoroughly flushed when putting another one in. Additionally, for safety's sake, you should flush the system when replacing your compressor this time. Find a new Delco compressor.
Additionally, I cannot tell where you are from. I'm not being nosy, I would just like to know the climate in which you drive. Bill lives in Kentucky and probably doesn't deal with too many 108 degree days. If you live in an extra hot area like Texas, I would not recommend changing this car to R134. You just lose too much cooling capacity, and will sweat for the first ten minutes after you start the car in the summer. I've changed over a few cars to R134, the 123 car does not respond well.
Best of luck,