You'll run out of editing resolution in FT long before you'll be able to get anything particularly useful. Assuming that you get lucky and FT will give you a 6000 resolution editing element, then you'd have 157000km per pixel or roughly 4 times the size of the earth per editing unit. Probably not terribly helpful. FT provides the option for a planar synthetic world, but you're still limited by the editing resolution. It would be a good solution for local areas (a few thousand km on a side).

I recommend a locally flat world. At roughly an earth orbit radius around the sun and with the very small width of the ring then the sun will be pretty much parallel from top to bottom across the width of the ring. Varying the sunlight would defeat the purpose of a ringworld: maximizing habitable area (plus, the angle would be constant - without seasons - so plants would be growing at funny angles from the ground as you neared the "poles"). I'm not sure what you're using for gravity, though, so a concave ring right yield some truly amusing gravity effects (possibly including the atmosphere all lifting off the surface).