I've done a couple of conversions and have had no problem with just vacuuming the system, adding the correct oil, and charging the system. I did a '69 camaro this way 10 years ago and it's still going strong.

My Challenger had a slow leak and I knew I'd be tearing it down for a resto this winter so I just charged it up with R134 each time I needed A/C. With the leak, it'd last a week or so but it was definitely functional and the stock compressor seemed to do just fine. I'll change the drier, condensor, and new o-rings while I've got it apart this winter but I plan to leave the stock compressor as-is. If it ain't broke, don't fix it.

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