Are you aware of the correct procedures for a reverse conversion? It will be well worth your while.

As far as the propane goes, I'm a purist. I personally would never introduce a flammable or any other foreign substance into an a/c system for ANY purpose.

It would be illegal to draw a vacuum as a gross leak check and then once it passes that, blow a little R22 in the system for leak checking reasons. The legal way to use R22 for leak checking is to pressurize with Nitrogen and then introduce a few ounces of R22. You can then blow the 22 into the atmosphere legally under those circumstances. The reason 22 is used is because it is highly detectable by a sniffer. Too bad it's illegal to just shoot some R22 in the system without the Nitrogen. The Nitrogen pressurizes everything to help with leak detection, but for your purposes, a pound or so of R22 would probably verify your evaporator as leak free. Too bad it's illegal.