I had a system converted to R134a and it worked ok. The guy had to change the throttling valves (which control the operating pressures in the condensor/ evapourator). I might even have a R134a chart that you can use to figure out what pressure to aim for. Book's at work though.

It's illegal to use in Aus, but propane has very similar Liquid-Vapour phase properties to R12. It's illegal I think because of the risk of propane leaking into the cabin and igniting. That said, propane has a much lower environmental impact to both R12 and R134a if released to the atmosphere. R134a still degrades the ozone layer, it just has a shorter half life than R12. They should always be disposed of responsibly.

Hope this helps.