Jerry has just the same thought i have. box it w/ holes as the mid section is from the factory. in other words keep the structural integrity of the box but have the steel company or you plasma cut holes triangles or ovals in the box before you weld it all up. you would have access inside the box for wiring, fuel and brake lines. some steel shops can do this work for you as in small job production.
my fairly local one does big and small jobs. and in this economy many more may also job it for you.
http://www.sk-steel.com/
