The Stovebolt Site Forums The Shop Area Paint and Body Shop Question related to welding and repair of rust

I'd say you'd be OK.Clean the area down to shiny metal. Go slow,let the weld kool to touch between passes.
Is the frame boxed or formed channel?(Edit;Did I read your Q. right?)
If formed channel try to stay away from the edge of the flange 1/4 to 1/2 inch to prevent it from cracking.Otherwise you can put a strap on the back side of the flange.
Have fun.