This is slightly off the subject but, if you are really getting 16-19 mpg, there should be lots of room for improvement with a tune-up and adjustment. I'm getting 22-25 in my "71 142E -- mostly rural back-road driving (45-65), 25% highway (65-75) -- and there are guys out there who claim to be getting as much as 28-30mpg, which turns me green with envy. If T&A doesn't produce a substantial improvement, a rebuild my be in order, which would, probably, cost about the same as converting to LPG/LNG, especially if you do everything but the machining yourself. From strictly a monetary point of view, considering the price of propane here in New England, the conversion isn't worth it. The environmental argument, of course, is an entirely different matter and well worth giving consideration.
Bob S.