The aluminum ones from most any auto parts store are JUNK! Go to aircondition.com, then click on "straight to the board" then scroll down and click on ACSource. They sell some brass adapters that are the absolute best quality for $2.98 each. I highly recommend them.
I more highly recommend staying with R12 on these cars. It is coming down in price at $29 can and will give you proper cooling.
Don't even consider charging 134 through R12 fittings. You are putting our remaining R12 supply in jeopardy when you do such a thing, not to mention causing someone else down the line a real headache.
If you insist on converting anyway, use the acsource adapters, flush everything, use green o-rings with Nylog lube on them, replace the filter drier, drain all the mineral oil out of the compressor that you can get, put in the correct amount of Ester oil, evacuate and charge. Don't settle for any conversion that shortcuts any of these steps if you want it to last.
And after you convert, you can forget about taking a sweater with you, because you won't be as cool.