At first thought you may not be worried about propane as a refrigerant, after all, it's sealed up in the system. But, what happens if a compressor siezes making a spark? Answer: the car turns into a bomb.
Did you see the picture of the bus in Israel about a month or so ago that was taken out by the suicide bomber? The top was blown completely out of it. That's what your car would look like if your compressor siezed.
So, I ask you, do you feel lucky?
Please don't take this approach.
If you insist on not using R12, and I understand the problem, VERY expensive, take the capacity loss and PROPERLY convert it to R134.