I worked for an oil company in Eastern Venezuela in the early 1960's. They had to burn off propane, but paid for gasoline. Consequently, they fitted most of our trucks to burn either gasoline or propane. There was a device on top of the carburetor to switch from one to the other as needed. We were firmly told to burn propane unless there was an emergency (no propane available). Worked pretty well, but occasionally had problems with the switching device.
Incidentally, the propane or natural gas is much easier on an engine. We had a pump jack running on an automobile engine, fired with natural gas, and it had the equivalent of 500,000 miles one it. Never needed new spark plugs or a tune up.