If that's regular hot-rolled steel tube, it's not likely to be flat enough to mount those profile rails on; they're very particular about flatness. It doesn't really matter which side of the tube you choose - none of it is very flat. People have got around that by building a dam on top of the rail and pouring in self-leveling epoxy to provide a surface flat enough for mounting rails on. Another approach is to screw flat aluminum plate to the rail, or better yet, Blanchard-ground steel plate, shimming where necessary.