Are we talking about the rear cab section below the belt line? I believe that there is a panel available for the entire back of the cab including the window hole.
Whichever way you go, there will be high level skills needed to pull that off.
The full panel would require a butt weld all of the way across the width of the cab at the roof. A nightmare for someone without high level sheet metal welding skills. The benefit would be no seam to rust out at the belt line later on, (The full panel is all one piece.)
With the panel for below the belt line, you will need to cut below the seam of the belt line and hope that the lip at the bottom of the upper section is sound.
One benefit of just replacing the cab corners is that if your work is not perfect, the bed covers up much of it.
