Sounds like an awesome trip! I always wanted to do something similar in a 40. I like the logistical challenge. Done something similar a couple of times in my 60. Once with a club (Thank GOD) and once alone. I look at traveling in these vehicles the same way I would a backpacking trip. Only with wheels. Which is to say I try to bring absolutely only what is necessary. I think Everything I pack in my 60 would fit in a 40. Heavy on life sustaining items (water, food, fuel/fluids) that can be containerized. I think all personal gear (clothing, personal hygiene, etc) should be kept in a rucksack or something you could otherwise throw on your back if you have to un-ass the vehicle and walk. A separate waterproof duffel bag for sleeping arrangements (tent, sleeping bag, etc). Cooking stuff gets it’s own container, and I pack lightly for that. I’m not Wolfgang Puck…
Spare parts (belts, hoses, various sizes of bolts, twine, spare gas cap, and a tie-rod end) should/could fit in a small/medium box or bag. People pack for every eventuality and end up taking too much stuff. Of course, you’ll be happy you had it if you need it.
Tools are a really touchy subject. Take what you need, but I’d be careful about taking so much that you need multiple containers for them. Obviously judgment is your guide here, based on your experiences.
I like roof racks for that kind of travel because it keeps my interior as free of detritus as possible, minimizing the potential for clutter and stuff flying around in an emergency. But only light stuff goes up top. Goes without saying everything inside the vehicle is lashed down or otherwise secured.
I pay close attention to GVWR as well. Road miles and even fire-road miles are NOT the same. Take a close looks at your tires and suspension. YMMV Good luck!!!
ps: please ignore the bungees on the roof rack. I know better now!