[PeteLeathersac]

Not too familar w/ Propane but think it's probaby cheaper to source the conversion materials than Natural gas is since it's more widely used in the Forklift industry. .

That said, I had some experience w/ bi-fuel Natural gas-Gasoline vehicles about 10 years ago and the biggie there is Natural Gas needs a much higher pressure storage supply tank than Propane so that's expensive alone unless you have an onsite supply source you can tap into. .
On carburated bi-fuel Natural Gas/Gasoline cars the carb can stay and the natural gas is mixed w/ the air supply before going through the carb. . The gasoline supply is shut off then the engine run until it's running out of gasoline and you flip over to the natural gas. .

As a side note, on engines dedicated to Natural Gas only you can up the compression ratio as Natural has quite a high octane rating but this is also the down side of bi-fuel system as the lower compression ratio used on the gasoline has less power operating on the natural gas even with the timing advance system used when running the Natural. . I always thought a dedicated Natural Gas only performance engine would be good as the octane is somewhere north of 125 octance?

Best advice would be to research a lot before you spend anything and if there's no onsite supply source of Natural Gas at hand, don't even consider it. .

~ Pete