Your idea has merit.I think you already reached the conclusion of "impractical to reengineer". (Probably why Ford is coming out with the diesel version f150 ?)
Ford Truck Blog » Blog Archive » 2010 F150 Diesel Spotted!
I spent a month in a country that was full of 4 cyllinder diesel trucks. The economy is great and good for towing smaller loads. When I returned , i looked for something similar here. The Jeep Liberty was one that I considered. Friends warned me against it.