Hi Bob,
Maybe just model a 320, you will not be far wrong, or maybe even correct. You have some dimensions for a 320, its wheelbase is 2880 mm, that is always a good start for scaling a vehicle, wheels are 6,50 - 20, which gives an overall diameter 33 inches (840 mm) another useful dimension, (happens to be the same as the 340 wheel size)..
The rear boot layout of these Reichwehr Kubels often varies , on some the bodywork ends just behind the rear seats and two strapped down storage boxes are then added, some have a single box, with room either side, maybe thats what you have here with jerrycans being located at the side of the single box, some later versions have bodywork that extends beyond the rear seats to become a "boot" with a lift-up or drop-down rear door.
These Reichwehr Kubels can vary a lot over time, even the same make and model, they were built in small order numbers, and for each small order, maybe was only for 50 vehicles at a time, so the next 50 would not necessarily be exacly the same in details. The other point is that these Kubel bodies were hand built, a wooden frame covered in steel, not mass produced pressed out identical metal bodies.
Remember there were only 41 of the 340 model made, thats no more than a pre-production run quantity for most auto companys, basically almost prototypes, variations could be rife across the 41.
cheers PAUL,