Hey Bob, yeah, the R-12 I took out then went into my '93 pickup. When charging R-134a I took John's advice of 6-7oz, ran it for a day, and everything checked OK. Nice even frost pattern and 'beer can cold' return line. Fridge and Freezer temps matched the GE service lit.
I don't have a nitrogen setup since I'm only a hobbiest; but this was a small enough system that I was down to 500 microns in an hour, and stick with Stay-Brite #8 for my joints. I've heard of some guys using welding gas as a purge, but it's a little pricey.