I don't mean to discourage anyone, but the reason I asked about what type of driving you were going to be doing was to make sure you were spending your effort in the right area. IE - if it's going to mostly be a highway machine, go for it. City machine? Won't make as much of a difference.
I think you're right to be concerned about putting the hinges on the cab. One reason: there can be a lot of flex & movement between the box vs cab, and it'll stress the structure if it's firmly attached to both. Best to attach it to the bed only.
You could add stanchions to the forward end of the bed, and fasten the cap hinges to them.
Another thought: while the aero cap gets a lot of attetion & discussion, don't forget about some of the much easier to pick low-hanging aero fruit, like grill block, air dam, etc. -
List of aero mods you can do to your vehicle