Propane conversions on tractors and farm trucks were very common in the Texas Panhandle in the late 50s. The manufacturers even got in on the act.
Of course it was butane then. LPG changed to propane later.
Clean burning, most modern engines are fine with the valves, and the ideas about higher compression or advanced timing are correct, too.
The actual propane carb is dead simple, just a rubber diaphram and throttle valve. On dual fuel systems, the diaphram carb was placed on top of the gasoline carb, and the gas butterflies regulated flow.
These systems would switch between gasoline and LPG without a burble. Some would even switch to gasoline to shut down and restart in the morning, as the LPG vaporizer worked best after the coolant water warmed up.
I have no idea how such a system would work on any kind of (gasoline) fuel injection or throttle body.
IF you go dual-fuel: Make sure and switch back and forth often. When the gasoline system is rarely used, the gas evaporates, leaves residue, and the gaskets shrink. None of the above is a good thing.
