LPG can be a good choice, however, to gain the pull potential from this fuel, it will be necessary to build an engine that reaps the benefit by design.
The simplest things to be aware of are the fact that a higher compression ratio than we are accustomed to will be desirable in order to release the full potential from every combustion. Something like 11 to 1 will suit LPG quite nicely, but this would rule out dual fuel operation.
Another is the fact that it is a 'dry' fuel & it offers no protection to valves or valve seats, so it will be necessary to ensure that both the exhaust valves, & the valve seats are compatible with LPG. No big deal, but something to be aware of at build up time.

You don't necessarily get more mileage from a litre of LPG, but the significant thing is that it costs so much less to buy that litre, so the cost per mile/kilometer IS significantly less.
Another upside to LPG, one that B210sleeper has already mentioned, is that oil changes are all but a thing of the past. LPG is also a very CLEAN fuel & oil contamination from the gas is almost non existant.

Those who reap the greatest benefit from it are those who amass big mileage, like Taxi's & commercial travelers. It's these people who allow their operational savings to pay for the LPG installation, then go on to bank the savings for every mile they drive.