Here in the Netherlands about 50%-70% of the American cars (oldtimers and new) use propane for feul, also lots of car with bigger gas engines use it. With using a regulater and a "mixer" on top of the carb is the easyest and cheapest but with LPI (liquid propane injection) you got as much or even more power then on normal gas.
but a LPI system is computer controlled and therefore difficult and expensiver on old carburated engines.

for example, higher octane pump-gas for our older engines is around $7.60 for a gallon and propane around $2.66 a gallon so you understand that it is populair here to convert to propane