I have been waiting for somebody to do a build on this platform. KUDOS to you for taking this on! I'll be watching this one. One thought is to height on an already super tall vehicle: have you thought of doing a hard sided pop up? Maybe a chopped down ISO container with a lid lift to keep the height under control? You are 12 high as is (about as high as most 1 ton truck campers out there).
These trucks are designed and equipped to transport standard-sized containers, so I'm expecting that it will be below the legal maximum height for on-road use.