It's not really worthwhile to do this for a non-commercial vehicle. A loaded tractor-trailor will get 5-7 mpg, and consume several hundred dollars of fuel per day. At that rate, it makes sense to spend a few thousand bucks for a
dual-fuel LNG/diesel conversion. If you can save a hundred dollars a day in fuel costs, the switchover becomes more attractive.
Switching to a gasoline based engine would be the cost factor. Do not think the math is there to produce a 600 bhp motor. And in a car, if you wish to give up 2/3 of your trunk for a fuel tank, well that can be done.