1-answered above

2-Not a backyard type installation and EFI, not a carb, is the starting point. There may be a way to adapt older ones.
People mistakenly think it is almost like propane, WRONG. It is a gas at 3600psi.

3-Option price on a new Crown Vic is about $5000, dont know what the parts cost.

4-$5000 on a new vehicle, surprise! = the option price. No credits for "retrofit"
because there is no way to certify the EPA mileage and emissions.

5-Fuel costs vary a lot by area, but average about two-thirds the price of gasoline.

My opinions are based on my brother's Crown Vic and Honda Civic, dedicated CNG cars, operated in the Los Angeles area.

Jim