Due to the fact that the gas filler is under the seat cushion on the WWII jeep, the MB XL seat frame is designed in such a way that the bottom of the frame stays in the original position. A pocket must be installed in the rear wheelhouse and the legs of the frame are lengthened so the seatsits back into that pocket.
Since this is not a problem for "newer" models due to them having side-fill gas tanks, if you wanted the same effect, you could weld pockets into the rear wheel wells and, rather than using the XL seat frame, modify your current seat by cutting the front leg tubes and sliding the seat back into the wheelhouse and then welding in a spacer.