I have a 2002 Dodge Ram Van with 5.9 and overdrive. It is a hightop conversion. It gets 11 mpg. Period. In town, interstate, towing a small trailer, etc. It has the aerodynamics of a brick and fuel mileage is limited by that. Tune the engine you have rather than worrying about a swap.
You should be getting better than that even with the hightop.
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