I think the rust factor begins with the quality of steel used in the frame.
and ends with the way the parts are welded on.
If you're referring to the parts numbered 13 and 14 in your pic, I would think that they can be butt welded top and bottom. I guess they are attached with rosettes, or spots? that would allow water to get between the mating surfaces.
You'll need to do something about the tension, and compression flanges though.