It hasn't really cooled down, but I got out there and did a lot today anyway.

(No closeup pic, this is too blurry) I had been dreading removing this cage nut to remove the broken body mount bolt. Now that it's done, I don't know why I was so worried about it, two small spot welds hold the cage on, removed them and started drilling out the bolt until I could remove what was left with a centerpunch. Total time: 20 minutes.
I set the pan in place and used spraypaint to note where the edges of the braces were. Then I could drill holes for spotwelds. I had help in the garage today so he was able to get some action shots.
This is one of the seat-mount holes. The brace has an attached nut, so I'm drilling this oversize to make sure it all lines up.
Pan in place, holes for spotwelds are complete.
Michael was kind enough to remove the rear bumper for me. We haven't removed the brackets yet though.
This is what I've got to deal with in the back where the bumper was. More cleaning.
I've started spotwelds on the outside edge.
which has become spotwelds all around. I've done some welding from the bottom, but not as much as I want to.
Welding this pan in isn't done, but I've had enough for the day. More next weekend, hopefully. I still need to get some seam sealer.