Keep in mind that your A/C system is a enclosed system and won't "use" freon. If you have to add freon at all, you have a leak. Freeze 12 is no beter than R12. They both damage the ozone layer, don't belive me, read the can. With freeze 12 you don't have to flush and refill your A/C system as with a 134a change over.
A 134a conversion (done right) will work as good as any R12 system out there. If you really want to freeze your but off, install a 134a condenser and drier with the stock R12 evap core. I have seen vent temps at around 30 deg. With 100 deg @ 70% hum. out side
worth the bucks if you live down south. (This setup has known to ice over the evap. at times, Just so you know)