That's exactly how i did mine! Really the only way to deal with rust.
And you can either weld a new peice of metal or a harder method would be to rivet in a new peice. Then either grind off the welding " peanuts" to a flat surface or straighten the riveted in metal as much as possible.
Some bondo, some sanding, then shaping , then smoothing, an assload of primer, i literally used 2 cans on both sides. apply, let dry, apply let dry etc.
Then paint, was as good as new
It's really very easy to do, just time consuming, but IMO my B13 is worth it.