I'm in the process of replacing just the lower portion, too. With guidance from a skilled bodyman, I drilled out the spot welds using a Blair spot weld bit. This is where the exterior of the valance forms the infamous lost valley with the interior piece. The weep holes get clogged easily which leads to a reservoir of water and debris, resulting in the very common rust out in this part of the car. Then he had me cut the rusted sheet metal just below the horizontal ridge in the valance and down, parallel to the seams between fenders and valance, without disturbing the seams. I'm waiting for him to come over and weld in the donor valance. I've been taking a few pictures of the process, so I'll post those when finished. Initially, I had hoped to simply grind out the rusted portion and weld in a patch, but the structure is more complicated than it appears, once you dig in behind what shows.