Well, I didn't get the box completed but managed to get the deck fitted and the wheel wells sorted out.
It was after carefully getting all the supports leveled up that I discovered that everything changed when the chassis was bolted to the body!(sigh)
Here you can see the separation of the box components to allow assembly and disassembly, something to keep in mind when doing these conversions.
The fit so far, bulkhead detail and sides to be added yet and then the shaping of the box at the tail-gate, something still to figure out.