[67c10rustbuket]

The frame gets its strength from the vertical wall of the C channel. There is absolutely zero vertical wall left and about that much strength... like mentioned above all you have is a flat plate. It wouldn't take much of a load hooked on the rear if the truck to bend that frame. You can plate either side all you want, heck even use 1/2" steel, it won't help one bit.
If the frame is straight the way to fix it in my mind would be to install a large step notch and therefore cutting out the old c notch section once welded in place.