While I have not, and have no plans to raise my roof, I can confirm that your sides taper inward from the bottom sill up. My Thomas does the same thing which is why I tapered my framing studs that mounted against the wall.
I'm sure someone out there has a solution to your question, hopefully they'll sound off soon. Again, I'm not planning on raising mine, but I'd like to know the answer to this too. (Ya just never know. . .
It seems to me that it wouldn't be that big a deal to me to straighten the posts after the dividing cut, but I don't know if the roof of a "tapered-wall" bus is narrower compared to a "straight-wall" bus.