The tool to use to remove the rust is the Wurth DBS 3600. This tool is VERY effective with different size wire wheels and materials, sorry don't have a photo of it, look it up.
Remove all rust, cut through to wheel well, if need be.
Click to view attachmentMig welder, run some beads, grind flat.
Click to view attachment And repair the corners. Weld in and grind flat.
Click to view attachment Large sections, under cowl and front fascia, headlight buckets badly rotted would be easier to replace.
Good luck
Mike