I have a couple of the Opel trucks, and wonder if the Steel cab, will fit the Opel Tankwagen? And vice-versa I guess. I'd get a late war wooden cab truck out of the deal as well.
As well as an early war gas truck.

I hope this is in the correct forum. The answer is yes, the cabs can be swapped around. The exhausts may need to be changed, per configuration, but I'm on my way to having a 1940-ish Luftwaffle gas truck.
I'll also get a DAK soft-top cargo bed truck. A little scratch work, and basic Eduard PE set goes a long way.

Having plans (plans being the operative word here ) to build at least a half dozen different configurations of the Blitz/Maultier, I've come to the conclusion after tons of research, that the cabs used can be substituted freely. Very minor alterations might be needed, but won't be noticed on the modeling/contest table.

John,
True. The Tankwagen to Blitz/cargo just changes the exhaust. On the gas truck the exhaust dumps by the passenger door, not out the rear. I am curious about some of the external stowage/uses, but haven't found too many pics. The Maultier ought to drop on as well, they just jacked up the bed and moved the "Mule" unit under it, and lowered the bed unit on it.
The box van looks the same for many uses as well, radio/office/medical ETC.

The few books I have on the Opel production series (Blitz or Maultier) for the most part simply explained it as having the cargo bed removed so the factory crews, as well as the field crews, could get to the frame in order to modify what was needed to install the Maultier drive train. Yep...the routing of exhaust would change a bit. As far as I could tell, the very same cargo bed went right back on after modifications were completed (unless the crews were modifying the Opel for something else). The tracks for the Maultier came from the Panzer I/II family. Stowage? Again...subjective. I viewed a lot of photos with different variations, some photos with dead on factory install all the way to individual crew/unit 'local' modified stowage installed to different locations on the frame (with more than one per side as well). Ad hoc by the crews times, or then again, standard/uniformed/factory applications...take your pick. It came down to pretty much what ever the crews needed or wanted. I would suggest to others out there: make your choice period correct and theatre correct...Google reference photos to help if you don't have 70's or 80's Opel books on the shelf!!