[Pyrotechnic]

Honestly, I think a gas motor for this situation is the wrong direction. I have a feeling the new motor you are having built will probably get 8 MPG or less when heavily loaded.

A Cummins 6BT swap would give much better power and gas mileage. Yes diesel costs a little more, but they maintain better gas mileage when loaded and they are an industrial grade engine, which means that motor is going to have a very long useful life.