Kellen,
First I would check the archives on this subject at
http://gerry.vanagon.com/archives/vanagon.html

    Most likely, a good (reputable, conscientious) A/C shop, would not put
any R-12 in your system before it was properly tested for leaks. Due to the
age of your system it is bound to leak so whatever they put in would not
last long. Also that much R-12 would cost you an "arm and a leg".

      Said shop would hopefully offer to convert your system to the cheaper
R-134. Check the archives on this subject too. Anyway you go, it's going to
be expen$ive. The hardest part is going to be finding an honest shop. I have
heard some horror stories , which lead me to buy a set of gauges and get a
Freon license.

Ken Lewis
http://neksiwel.20m.com/