Back of cab, to end for rear frame rails on a long wheel base (127") truck, is approximately 95.5", but I measured several and it will vary +/- 1", depending on the mounting location of the cab. You may have to get a bit more scientific, with your measurements, whereas, bobbing a frame of a longer wheel base frame, rear of the rear wheels, will not render the same frame configuration as a factory long bed frame.
May I suggest putting the long wheel base bed on the frame, situated over the rear wheels, as you see fit, in order to determine the section to be removed from forward of the rear wheels and rear of the rear wheels.
