In my opinion you are wrong in thinking the smaller motor will save you money.
Yes! You may get a few more miles per gallon running empty, but the smaller motor will have to work much harder, when loaded therefore costing more money, in fuel, for repairs, and maintenance later on. If you are going to buy a vehicle to do a job, why buy one that won't fulfill yours, or your carrier’s needs overall.
I have been running vans for 17 years. Gas is the way I decided to go. WHY!!! Well for one, the gas motor is allot less$$$ and much cheaper to maintain. Gas motors are easier to work on, (I do my own work) therefore saving money over all.
I average 17 MPG running the (2002)=5.7, and 14 MPG running the (2004)=6.0, Chevy’s my personal preference. I did modify the exhaust, running duals from front to back, adding a second from the 'Y' back. This helped fuel mileage by about 3MPG. I just don't like the Fords, as they are small in the cab.