Warning ! Warning ! Warning !
Be aware these trucks had extremely poor jig work when they were built so don't expect the doors to both look good (gaps & alignment) and close well ~ you'll get one or t'other but rarely both .
Best bet is to tack weld the doors in place _before_ you remove the cab off the chassis , then invert it and repair floors , dash etc. and then once it's done , cut the welds and work the doors as a last item . this helps the cab retain its shape as you're working or welding on it .
The guy in my shop who has a hot rod '49 , has terrible door fittment , esp. the passenger side door as it has a big gap between the 'A' pillar and the leading edge of the door , this is the # 1 most common fault in AD trucks and it will leak if the truck is left out in the rain , his $60,000.00 + show truck obviously never gets wet unless he's washing it but it drives him crazy nontheless ~ he removed the door skins and re-worked the inner door's frame as best he could then re-attached the skins but it's still not as he'd like it .
Anyway , I wish you luck with it , I just adjust for easy opening and closing where no SLAM ! is needed and let the reveal mounding line up where it may , that's how GM did it .