Just my opinion Glenn, but attaching patches over the severely rusted out sections with screws and some seam sealer isn't going to give you any lasting and necessary structure to your cab. It might look ok standing still, but when you start to drive it, things are going to start moving/flexing and coming apart pretty rapidly. I would say X2 for the previous guidance on replacing and welding in new metal after cutting back the rotten stuff. Others have suggested that when you fit and position the new panels, temporarily using screws to hold things in place before welding is a good idea as it allows you to double check fit of adjacent parts, doors, etc. before welding to restore the strength and form to your cab.
Using screws to hold things together permanently is a recipe for disaster waiting to happen.
