I dont think its illegal, but I couldnt say for sure. I am sure it depends on the state. However, I would fab something else up, I refuse to put it inside my rig and lose any cargo space. A friend of mine just built a buggy and built a custom fuel cell. The steel supply bent the major pieces for him and the bottom, front and rear is one piece of 1/4" with thinner sides and top. Fuel cell with a built in skid plate.
As for the body, I was thinking of cutting away some of the body, moving the axle back as far as I can and then getting some Ranger bedsides from the wrecking yard. That way I could keep the factory look of the sheet metal but move the entire fender back. Most people probably wouldnt even notice the stretch if they were not RBV folks.