I've got a couple of cans I'll sell you cheap... Oh, I'm sorry officer !
In all seriousness, there is another option to consider. There is a product I use in all my old vehicles called Freeze12. It'* $8/can, and uses R12 gauges and fittings, but you will need a vampire tap (they sell it cheap) as the cans have a unique fitting. It really works well - it doesn't get as cold, but cools-off the car more quickly if that makes sense. Much better than a typical R134 car now, and infinitely colder than you'll get with a dump and fill 134 conversion to your bville. You can add it right to R12, but I would whole-heartedly rec. pumping your system down to make sure it'* tight, then use Freeze12. It'* available at Advance and I'm sure other places. Here'* the link to their site - it has all the info you need, plus a link to the cert. test if you want to get your 609 cert. and be able to buy R12, too. I have no contact to them or anything! I've just used their stuff and have been VERY satisifed.
e-mail me if you want to pursue this and I'll give you details of my experiences.