The back pedestal section where the backbox bolts on was separating from the cab, and here is how i chose to repair it. It was late at night when i did this, so i used what i had lying around, ie glued and screwed a piece of ply inside the cab, glued, clamped and screwed fro the outside, as can be seen in the first couple of pics. I filled the screwholes in the back later on while i was filling dents and gouges in the cab.

And last couple of pics of after all of the repairs had been done, and had been sanded back smooth with 80 grit, 160, 280 and finally 400 grit ready for undercoat.