These tires, just like some others, have a natural crown to them and I think even with wide rims you'd still have it to some degree. There's about no way I'd be able to drop the pressure low enough on my Michelins to get rid of the curve even with 9" wide tires on a 6.5" wide rim.
Here were my RT/II's on the GM OEM steelies:

Here were my RT/II's on the GM OEM steelies:
even with runnin 25 psi they still donut!