Nick

Pretty common problem. There are no repro rear paneltruck anything available, except rubber and tail lights. Luckily, most of the metal back there is fairly flat or only curves in one direction (as opposed to compound curved like the roof). If you find a paneltruck with good rear sheetmetal, use that one!

I've been on and off patching my 50 together and you will find more as you start removing things, especially in the floor and door areas, but don't give up on it.
How are the rear doors? These too, are really hard to come by.
If you are updating the interior, I am still on the look out for a passenger jump seat, if you have it as it was an option. If you aren't using the driver seat either, pm me, I may take it off your hands.

Good luck and take lots of pics, helps with the rebuild and keeps ooglers on the board something to ponder.

Chris