Just filling in the window holes as Ian has done is a good option as long as you have an experienced welder to do the job as the large panel will warp easily.... However in your case Blocker you have severe rust along the bottom seam and the top of the rear quarters will be rusted through.... this will need to be repaired and will be much easier and a better job done if the side panels are removed.

I am in the same boat, the right side rear window was leaking and it has rusted the back section of the side panel bottom lip and the top of the quarter so to do the job properly I have to replace the entire side panel.... as I'm doing one I may as well do both the same way.

I just need to find a set of side panels.....