Why can't you use a turbocharged 4 cylinder? GM makes them, and they were quite strong. There's a turbo 2.2 good for 260 hp in the new Pontiac Solstice, and the Olds Quad 4 engine was a turbo motor that made quite a bit of power too.
Thing about a turbo motor is, keep out of the throttle, and it's just a regular 4 cylinder motor. But you know, if you under-power a heavy vehicle, not only will you have the aggravation of being so underpowered, but also, your gas mileage will suffer because of how hard the motor needs to work.
Just a suggestion.
