Look up towards the top of the box from inside the wheel well. You'll see a wide flat piece of metal holding the box side to the inner box. Cut it length-wise, and use a 2x4 along the outside fender with a bottle jack against the inner to 'pull' the bedsides out. Then use your prefered method of reinforcing the area you cut, by installing a spacer to hold the newly wider boxsides stable where they now sit. You may also have to extend the rods going to the corner of the wheelwells. I'll get pics when I get a chance if someone else doesn't post any first.