yeah - my advice is to cut around the rust, and leave a short lip around the patching area (which i did not do). Next, instead of butt-welding, lap weld the section in. The lap weld will be much strong than a butt-weld, PLUS its 10x easier to just slap the piece down and weld. yet..from underneath, the lap weld does not look as good. I ended up welding twice, once from inside, and once from underneath.