sorry if missed that before, I thought you were looking to build something like a Tatra 815 or a MAN KAT.
If you plan to use it for off road touring like the video above, that changes a few things. This is what I would do if I were going to build something like that, so take it for what it's worth.
Buy two pairs of re-conditioned axles, done by a reputable shop with good parts, but I would not get into upgrades on the axles at all. Drum brakes are heavy and trap mud and water so they will need lots of service from the off road use, but you are going at slow speed so you should have no problems stopping with eight drums. I would throw on a couple of driveline brakes for emergency use or parking brakes. A disc brake conversion can be done on a budget and that would be my choice if you have the cash for it. There are tons of shops and vendors for the 2.5 ton axles. Anything you "could" want to do has been done by the mud guys or the rock crawling crowd, so you can go crazy if you want, but I see no need for that in this application.
I prefer a fully boxed frame over C-Channel, you want your suspension to flex, not your frame. Leaf springs are dead simple and reliable. Use a stock or similar to stock setup with a good number of leaves removed since you will not be carrying a huge amount of weight. (Thirty people at 200 pounds each is only 6000 pounds.) You can add a track bar to control axle wrap if required (from reduced leaves) and an anti sway bar to control body roll and make it a bit more comfortable for your passengers. You should not need huge articulation, but the anti sway bar may limit it some depending on how its setup.
If you are not going to drive the truck on the road, I would do a 12 valve B series Cummins that has been mildly modified, no need for crazy high HP in this application. You want good torque and good area under curve for all around performance, smooth throttle response, etc. If you want an auto, get one from Suncoast or Goerend Brothers built for your application and you will never kill it. An NV271 (manual) or NV273 (electric shift) transfer case will be fine in this application and an NV241DHD would probably be fine too honestly.
Build the cage however you want it to look and feel and weld it or bolt it right to the frame rails. This will give the frame an increase in rigidity and reduce flex. A 2" DOM tube in .120 wall should be fine for the main cage structure and you can use 1.75" or even 1.5" in non stressed areas.
An off road suspension seat from PRP or Mastercraft will go a long way to making the ride comfortable, even with just a lap belt, though I might suggest a four point harness.
With full hydraulic steering you can easily tie the two front steering axles together and could even counter steer the rears but that can be quite difficult for the operator, so I would probably stick with two front steering and two solid rear axles. If you feel like you want to tackle more aggressive terrain, I would throw in at least two ARB lockers or a Detroit/OEM style. With open diffs all around your eight wheel drive is really only four and that may or may not be enough depending on what you are doing on the trail.
I would finish the rig with things like heavy powder coat, spray on bed liner, POR-15, etc.. With the harsh environment cleaning the rig and preventative maintenance will make it last a long time. For the cab design I like the cabover style with the engine under you or behind you. Especially for off roading this gives the driver a great view and they need not worry about having a front end/nose to clear or hang up on obstacles.
I would take design ideas from other similar rigs like MAN KAT, HEMTT, TATRA 815, KAMAZ, Unimog, Actros (
Actros 8x8 and a 6x6 Very Nice! Looks very capable for a large truck! - Benzworld.org - Mercedes-Benz Discussion Forum) and the like. The driver could sit lower, kind of over the front axle to see the trail better, where the passengers could be raised up like in the HMMWV. For wheels and tires, the rockwell pattern is very common and a 20" wheel with some 38 -42" off road tires will be fine. No need for huge heavy tires for the terrain that you are going on. Continental makes a great MPT in a 38" version and a 41" version IIRC. They are true off road tires that can be aired down for off roading. A bottom diff mohawk with any number of kits on the market will give your axles the ground clearance of a Dana 60 if you are concerned with ground clearance.
EDIT:
my new all purpose modular 8x8 rig - Pirate4x4.Com
EDIT2: you could start with a chassis like this:
which would give you frame rails and a driver's cab. Cut the back of the cab out like a van cutaway, sell off the box, engine, trans, axles, etc to someone like Doug Busbee and build your cage on the existing frame. It could save you some time and money, as this one is listed for $7,000 in Arizona. Doug may sell you a wrecked one even cheaper.
Two pairs of axles should be about $3,000, wrecked Dodge Cummins truck for a few grand (take the driveline, sell or scrap the rest), wheels and tires (??), add your tube chassis and seats and you have all the majour components that you need.
What is your budget and timeline on this? Is it something that you are serious about constructing for your tour business?
That's the kind of person I am. If I am met with opposition, I only push harder.
