I looked at a propane conversion in the 70s. Once the cars got Stellite valves there was no good reason not to, except the lack of filling stations and the cost of the conversion (over $1000 in 74). Right now I am not sure propane is that much cheaper than gasoline. I don't think there is any LNG around here. When I was in Md I had natural gas at the house but not the compressor to make it liquid.
