There are some very good kits out there. In fact, there's a good off road company here in Tempe that sells them. But I don't know of any that are emissions compliant. Next, since you can't use fuel injection, what would you do with the PCM? You need that to operate much of the vehicle. How can it do that with no fuel injectors or sensors that will either be missing or inop? I'm sure that's why I've never seen an aftermarket propane conversion on a late model.
Next, if you do get it to work, the mileage is not as good. The amount of btu's per gallon in propane is quite low compared to gasoline. (appx 91,500 vs 113,000 depending on blend, temp, etc) Therefore, you need so much of it. However, it's generally much cheaper to buy. Now, the real beauty of propane is it's octane rating. It's in the 104 to 105 area. What this means is you can build a dedicated propane engine. I built a guy a 460 at 12.5:1 for his pick up on propane. That gave him back the power and a lot of the mileage you normally lose with this conversion.
