I run an `70 Fury Gran Coupe on LPG(Propan/Butan)for 8 years now.
It got an old Impco Ventury system, where you can switch between petrol and LPG.
A little less power and 10-20% less mileage on LPG.
Here in Germany 1 litre petrol is 1,60 Euro cent and 1 litre LPG is only 0,68 Euro cent. Haven`t run it on petrol since
Pros, apart from the price, clean oil, clean valves, clean plugs, nice smell
There is more heat to the exhaust valves, should be considered when you attempt to rev it high for a long period, without hardened seats.

Greets Arnd