Just get a custom cab made (as mentioned in my little cost estimate
). I'm sure someone on here must know someone who does it.
I don't see any advantage to an F350. I think if you really want the "camper" on the back there's a lot of superior platforms that can be added to. And I think with that setup you'll DEFINITELY be short on space. If you didn't want self sufficiency I'd say no problem take a BJ60... or a pink cadilac. But if you're not planning on staying in hotels the whole way, then sleeping and carrying that many people, PLUS your gear which takes up a lot of space. Basically I don't really think the campered pickup gives you much if any space advantage over a 60 with a roof tent. With that weight at that distance in those conditions, your F350 is also going to fall apart.
Old army trucks can be had cheap too. I think they sometimes have PTO winches. Maybe finding out if there's something you can get that's in commercial use in South America for parts availability would be a good idea.