My recommendation would be to reverse your steps 4 and 5. Replace the rocker panels before the floor panels which would make the door openings rigid. Rather than depending on the FAM dimensions, I would put your bracing in before removing the doors like clarsen3100 said. Using the margin around the doors for body alignment would be more dependable on the dimensions, as your doors are unique to your cab, and would get you closer when reinstalling and aligning the doors on re-installation.
I'm at the point on my cab of installing the rockers, but I only need to fully replace the driver's side rocker. I completely removed the cab and put it on a rotisserie, which does dual purpose of aligning the door openings and providing pivot points. It gives me better access for repairs. I'm getting to old to lay on my back to do welding.
Our earthquake put a hold on my progress, though. My town was the hardest hit, and some local seismometers put the magnitude over 8.0 as opposed to the published 7.0.
Sounds like you're in good shape overall. Good luck with your restoration.
