I've got what you need here at my place but you would have to have a buddy with a truck that comes down to the Yakima Valley of Washington state to haul it back for you.
That is a real straight and solid back panel of a 53 truck that is cut off though the middle of the rear window and that I am going to cut the cab though the floor somewhere to use the front half for my stretch cab's cowl. My buddy cut the roof off probably 30 years ago and I have had this thing stashed away ever since. There is one dent in the belt line on the passenger side that has to be fixed. You could use as little or as much of it as you wanted to.
That meaning to me that the best way to fix it is find the back section of of a donor cab that is in real good shape that you can use as much or as little of it as you are comfortable with. Then cut the outer sheet metal though the rear window just as if you were chopping the truck and cut and fit and weld back.
This is the 51 cab that I am using for the back half of my build with cut line. This back half gets welded to the front half of the blue 53 cab. The actual simplest way if you found a real nice back section of a cab is cut the cab through the window and cut across the floor and replace the whole back section. That would keep the finish metal work to a minimum especially if you popped the inside cab panel out before you cut the roof.
