That is an interesting question. I think when the question gets answered, we find out if the box is built on the chassis of the truck, or if the box is built separately and then put on the truck.
You might want to try Marion Truck body in Marion, Wisconsin; west of Green Bay and north of Appleton.
Marion Body Works
Talk to them about building you a box, take your truck there, bolt their body to your chassis, take it home, and finish out the inside.