For an R-134A conversion, Four Seasons part # 59978, remove the valve core from the original (make sure system is empty first!), and screw this one on. It comes with a replaceable valve core.
The original is 3/16", and the original valve core is GM part # 10027604, or AC Delco part # 15-5375, if you were keeping it R-12.
You also need a special larger valve core tool to remove the old one, Four Seasons # 59302
The other end of the tool works on 1/4" ones too, so it's not just a one-time use thing. Also works on tire valve cores.