Sounds a lot like the british (maybe?) company that was advertising some huge mileage in a small car witha huge deisel engine... there was a post around here recently. I don't see why a huge engine turning 600rpm at 60mph would use much gas... seems like a feasable feat of engineering to me...
My own recommendation would be to put that beast in the front of the bed with a shorter drive line. It would put the weight in the middle and open up the front for a "front trunk"... encase it in a kammback, too... I'd haul my race car with it
As for the mpg and hp figures, it is SUPER common to chip a truck so that it can be set to high hp OR high mileage... so they dyno it on the hp setting and cruise around in econo mode... it isn't that outrageous.