Hi Probird,The rear cab panel is made up of 2 peices.
The upper and lower, which is separated at the belt line.
The upper is actualy the roof, and includes the belt line & overlaps the lower panel. The lower panel can be removed if you drill out the spot welds.
The spot welds run up the door jambs, across the bottom where it attaches to the floor, & across the belt line from inside the cab.
It might be worth it to replace it with a panel from a donor cab.
I was considering doing this on my truck, if not, I will remove it to repair it, then weld it back on.
Mine has a little bit of rot at the belt line.