Just out of curiosity have you thought about a diesel swap? A 6.2 or 6.5 would definitely increase your mileage, and you could use bio fuel ie French fry grease.
given the price of diesel, im really concerned with the cost of the conversion verses benefit. im not sure about the bio diesel stuff ill research it some more but it seems the cng or propane is much better payoff in the long run. just my opinion. but on second though hmmm you got me thinking.