[Big Dave]

Check out the garage and the aerodynamic forum. Basjoos is the poster child for aerodynamics.

6.2/6.5 engines will fit anyplace a 350 will go, although you'd have to forego the turbo in a cramped van engine compartment. The Cummins is smaller, turbo and all and can be turnedup to make a lot of HP if you want.

We all have to balance acceleration vs MPG. Roll your own. GM built a 6.2 Suburban with a 2.73 and allegedly got 30 MPG, but it was real slow. 3.08 might be a lot more do-able.

MPG is like any other parameter of performance. The old hot-rodders' saying of: "Speed is just a matter of money. How fast you wanna go?" applies. MPG mods are just a matter of money. How much MPG you want