Kevin nailed it. Bracing.
The only other thing I could think of is to first replace the door hinge pins. Go oversize if needed. It is common for the driver's side to be severely worn.
I've seen a bodyman have someone to raise and lower the door while he closely watches hinge for play.
Once you have the lost motion out of the hinges, you can do a better job of door alignment.
The striker and latch alignment works better as well.
I remember my uncle would chew me out for leaning against the door of a moving 1950 1 ton truck.
Even with good working 47-51 latches, the doors historically can pop open by themselves (you need safety belts).
New preproduction parts are worse plus reports are they have alignment issues afterwards.
The cab is bolted to the drivetrain frame on cushions. The frame is designed to move/twist which means the cab can flex as well.
