I did toy with building a demountable on a double cab!As I a pilot, I am aware of the load placement and the affect on the center of mass (gravity) and handling. Ford publishes a separate body and chassis guide for the Ranger that shows the relevant information so vehicle converters can properly design and attach conversions. The information in this manual is similar to the aircraft manuals I used to calculate weight and balance. Sometimes I shake my head in disbelief when I see how some drivers load their vehicles and trailers...or aircraft.
I have the Super Cab, not the ubiquitous four door Crew Cab, and the Ford recommended range for the center of mass is a zone that starts at the rear of the cab and extends into the bed for over one meter. I wanted the standard cab, which provides a larger CG zone in the bed area, but the only models available without ordering were the 2WD version with the 2.2L manual transmission in white. On the Crew Cab version, the recommended CG zone starts inside the cab at about the middle of the rear seat cushions and extends rearward for about one meter, so the recommended CG zone ends about 800mm behind the cab. I bought the 4WD Wildtrak with the 3.2L 6-speed automatic transmission. It is more comfortable than the 2006 Mondeo I owned and has now become my daily driver.
I know some owners in the States modify their vehicles as you described, but this is not allowed in Germany. Unless the modifications include a TÜV certification, they cannot be installed and the penalties when caught start with loss of the vehicle and go up from there.
Just as much space as my van but a challenge. Instead I've a transit with welfare facilities.
The 3.2 is better than the dpf troubled 2.2!