[MP&C]

Those rust spots are typically more widespread than you see from the outside. There willl be more on the back side not yet broken through. I would suggest using an ice pick on the ends to see how far you need to go and I would also agree that one piece would be better. In this case, as you are welding a concave shape (both directions) any shrinking is going to pull the weld seam outward. You may be able to use a crowned body hammer and an air dolly to restore the weld to its proper location. If you’re lucky.