I replaced the R12 in my Toyota with Freeze 12 (don't know if it's still available though) which is an R12 replacement and is just as cold, if not colder than R12.
You can't however, just add it to an existing R12 charge, you have to purge, evacuate and add the proper oil before charging.....always replace the accumulater with a new one.
I didn't use the conversion fittings as I was the only one working on it.
I found that R134 didn't cool as we'll because it isn't as efficient at moving heat and my system was sized for R12.....R134 systems have condensers and evaporators sized accordingly (bigger) also, since R134 runs higher head pressures you should install a high pressure cut off switch.....I didn't and blew the seals in my R12 spec compressor....thus my switch to Freeze 12 and no, it doesn't contain propane.
Just sharing my experiences.
Hans