Hello Quintin
The Mog gearbox and transfer case are one unit with the reduction and the reverser at the end, giving eight forward and eight reverse gears. It’s also a structural part of the suspension, being the anchor point of the huge triangular swingarm. Merc also make a bolt-on transfer case for “ordinary” 4x4 trucks. It doesn’t mage your G4 any less cool, with 14.5x20 tyres it must have an excellent attitude. Now the search for two trucks with the right CW&P will begin. With the help of the CD and a telephone you’ll root out what you need.

My Iveco’s nothing as interesting (or as high off the ground) as your G4. Mine’s a Turbo Daily 4x4 modified.
The chassis is the 5 tonne NATO reinforced with suspension to match, it runs twin wheels on the back and the camper box is 4m long, with an extra 1M50 over the cab in motorhome style.

It’s woefully underpowered, and in France where the Raid culture is king there are plenty of hop-up kits and tuners which/who propose to add up to 30% power by fiddling with my Bosch injection pump. One day I’ll do it, but only after a careful study of supplementary cooling available.
The intercooler would help that, but not by much. A TD engine already receives more than the necessary air into its cylinders (as a heating agent to set off the explosion) if the charge of air is cooler the engine runs cooler and can therefore burn more fuel. Adding an intercooler without boosting the injection doesn’t do much for the power, but helps the longevity of the engine. One of the truck rally teams would be able to point you to someone who could soup up your pump.
I don’t have air brakes, so the full length between cab and back wheels is tank. I have custom welded alu tanks simply bolted through the floor of the box with big reinforcing plates. They give me 260 l in total fine for the little engine that only consumes 15l/100km. Behind the rear wheels on one side is the original Iveco tank which serves the generator, on the other side. (It’s petrol, I want to change to diesel to get an extra 80l fuel capacity)
I’m sorry I can’t give you the name of the tank manufacturer, I bought the vehicle 2nd hand; It was made by MachZone in Hull though.

Hanging the tanks off the box is crude, but it gives more capacity than a chassis mount and reduces the vibration they see.
I’m sure you could put your air tanks on the inside of the chassis, there’s usually a lot of space between the chassis beam and the driveshafts that is badly exploited. A bit of canny plumbing is all it would take. Or perhaps behind the rear wheels.
It’s worth keeping the tanks low for stability reasons, your space between cab and box is better used to store sand ladders (less visible) or to cut a passage through.
I would recommend getting the batteries out of temptation’s way. I don’t know how your box is laid out but if there’s a bench/dinette up front just put them under that, directly above the original position (in a plastic tub). The cables could even be long enough to accommodate this, if not a welding supplies shop can sort you out with the necessary.

Taking it easy we once managed 1940 kms on tar between fill ups (the last 100 were nerve wracking) A range like that is very rarely necessary but it does allow you to be choosy about where you fill up, and to take advantage of cheap fuel when you find it.

I envy you; I’m head down at a boring job dreaming of the next escape (or how to pay for it more precisely)
Happy planning

Luke

(The length of the text is directly proportional to the boredom of the job)