I would recommend cutting out the corroded section of the bottom flange. Weld in a patch, and grind the welds on the inside flat. Then weld a doubler on the inside of the frame, going a few inches past each end of the patch, if possible. I'm assuming this is in a straight section, not where the frame humps up to go over the rear end. If it's in the curve, the fix will work, but it's a lot harder to do. I would do the welding with a stick for best penetration.
