You could section it either under the cab or sleeper back and it's not too bad to do one rail at a time. You have a pattern this way and be certain to measure the distance between a hole in the part of the frame that remains, to a hole in the frame for the replacement rail to start as a basis for your measurements. After you install the new rail, drill your locating hole in the correct measurement as before and this is your starting point.I have a 1980 359 that needs a new frame. I was wondering if it would be easier to put anew frame under it or get a 379 rolling chassis to the body on. Would it work on a379 chassis?