There are several things to consider....here are a few that come to mind
Legal maximum height is 13' 6" (it may be 14 in some states but I wouldn't suggest going over 13')
Are you using a light duty chassis or a class 8? How high is the top of the cab? A class 8 Cab roof is about 9' high, (for comparison that Uhaul truck in your pictures is probably about 9-10' at the top of the box!)
Are you going to cut the back of the cab out (leave the top intact), then put your box behind and over the top of the cab roof? In this case you'll want a flexible rubber seal to allow the box to flex differently than the cab.
If you use a class 8 truck you won't get much more than 4' max height in your room over the cab. You are also limited by how far over the front of the truck you can go. Your cab will be 4-5 feet long. If you go much further over the cab you will likely have problems with the hood not being able to open properly, (and it'll look strange!)
You could gain a few more inches by cutting the top of the cab out but you can only cut down to a couple of inches above the windshield and that brings a whole new set of limitations...and problems involving the way the box moves with the cab as the truck moves and flexes over the road.
If you're planning cutting the roof out of the cab and rigid mounting the box you'll be limited to a fairly short box (aprox 14' or less) and want to use something like air ride/air bags to allow the cab and box to remain rigid while the chassis flexes underneath.
If you are going to use a light duty truck that is lower to the ground you are now limited by the chassis and what size box it will support.
What are your goals for the vehicle and how will the room above the cab work with the rest of it?
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