[Auptimist]

The blue curve section should be near 12° but I wouldn't put too much faith in those templates (don't fight me on this, guys, I'm sure they're great...). I would just measure the cab height and do some trig. I don't want to sound pretentious at all but here's the procedure to do it by hand if you don't feel like measuring a bit of CAD (which is what I did considering my goofy box sides):
To get the vertical displacement of the end of the shell (truncating the eave/overhang) from the rear of the tailgate:
h_rear = h_cab - box_length * tan(Θ)

From what I understand from other truck threads (I think I've read most of them at this point), the measurements of the simplified flat-profile shell also work for those that are curved to be tangent to the cab roof. Allegedly, so long as you follow the Kamm principle and don't round the rear edge to be tangent to the back of the tailgate, you can get away with a convex shell without significant flow separation even though it clearly is more than 12° past the midpoint. Like I said earlier, I am not yet convinced convexity is worth that flow separation risk and extra effort?

Still definitely curious about your frame choices, as to diameter and EMT vs. Rigid or IMT.