In my r100 7 inch chop I just marked with tape both cut lines (roof and doors still mounted) and triple checked. braced it up big time with rebar, cut roof off, cut roof in half again above passengers. The vertical pillars from the door were reused (modified) as the horizontal door window pieces that are now needed because front of roof is lower and more forward than before. The same goes for the a pillar pieces ( reuse them to extend the horizontal roofline at the drip rail). Basically as you chop the roof and reset it you now have a butt crack in the middle to patch up , cover with a soft top , , sunroof style it, or do a proper metal patch. Other option is to v notch base of a pillars and bend them rearward and NOT cut roof in half in the beginning. I think if you look close at your back of cab that it is not vertical. the cab roof will move rearward slightly no matter what - causing you to be creative.
