I believe that the back of the cab wall had either had vertical wood supports or horizontal. I think the seat back cushion attached to them. See Link.
The cab corners look almost complete as far as metal loss due to corrosion. Most are rotted completely out. I would remove any corrosion rot there and fabricate butt weld patches. Only remove what is not solid and replace with same gauge of metal. Cab corners are available but yours are definitely salvageable and they already fit.
https://fordwood.com/images/rpu28new.jpg