Keesje,
If you're working on an update, may I suggest you start looking at a freighter? The basic dimensions of the floor would probably accomodate a standard 125 inch (318 cm) by 88 inch (213 cm) ULD. Try to make sure you can have around 220 cm of height clearance over a width of around 100 cm at the roof. That will allow loading of standard AAA type ULDs. Also, ensure there's room for a restraining system by placing the ULDs with around 1 inch of space between each position.
Cargo door should be behind the wing, wih dimensions of around 220 cm high by 340 cm wide.
If you can make all that happen, and add a couple hundred miles more range, you'll have a feeder par excellence for the integrators on your hands.