1977 Ford Ambulance w/91 straight six

[rvlifenow]

I acquired a 1977 Ford Ambulance dually really cheap. It had a 460 big block and a C-4 in it. At the time I ran a shop and acquired a state owned 1991 e150 cargo van with a 4.9 300 straight fuel injected six along with an overdrive trans recently rebuilt by the state. The 91 motor and trans were perfect so I had my mechanics transfer the entire thing along with harness, etc into the ambulance and sold the big block and c-4. The entire switch put me into a situation now that the ambulance is free. I am customizing it into a weekend overnighter Jimmy Buffett, Margaritavile rat rod. Here is what I would like to know. The fuel injected six, with the overdrive trans does not get any better gas mileage than the big block. This does not make sense to me as the ambulance is completely gutted and the only thing left on the back is the light aluminum box. I mean it is totally gutted. Does anyone here have any ideas which auto trans would bolt right up to the 1991 300,4.9 that would have better gearing or a two speed axle or something where I could cruise down the Interstates a little better? Right now I am getting about one driveway per gallon.