A general rule of thumb in rust repair is if you see a pinhole, then the area you'll have to remove to get to fresh usable sheetmetal will be the size of your palm. This was relayed to me from a lifetime auto body tech, and so far my experience with rust repair he's been spot on correct. If you cut corners here, the rust will be back within a year or two if driven in the rain regularly.
When you start cutting that apart, you're gonna have a nightmare on your hands. It will take 3 times longer than you think, and the rust will extend further than you ever thought. The metal will be thin a long way away from the rust you can easily see.
If you just plan to cut yours out and weld those on, you'll only be welding the outside sheetmetal together. Purely a cosmetic repair.