Which engine is Cooler?

I'm impressed with all the great ideas you guy's are coming up with on how to spend MY money. One important thing to consider is MPG. I plan on driving the wheels off of this truck, hopefully everyday in the nice weather, so fuel mileage is an import consideration.
I know the Hemi will do around 20mpg, the Viper probably mid teens ( the truck should weigh less than 4K lbs.) The old school 440 could do mid teens if I put fuel injection on it. So right now the Hemi is in the lead.