I've seen ladder bars located under and inside rear frame rails. Mine is new 2X3 frame construction and I want to locate bars and coilover shocks outside the frame so they may be seen. I've altered the rear wheel base forward so there is no room to frame over bars. (seat clearence). Inside tire width is 40", ladder bars at 33" center, which would put frame at around 29" center. I'm sure it's been done but I haven't seen it.