There is a difference behind the wheel as well...4" shorter.
Simply need to cut 16" (115 "wb from 131"wb = 16") from the frame rails between the front box mount and the start of the rear axle rise. then section the box similarly. For the rear frame rails (behind the axle) lop the frame off at the forward most bumper hole then remove the next 4” and weld the bumper section back on.Then section the rear of the box 4" from betwene the wheel tub and the rear box mount. More body work than frame work.