So I am a bit overdue for an update. I have made slow progress, with lots of hours put into fixing the rust and reworking the replacement sheetmetal.
I put in the patch that I made above, it fit really well. I welded both sides so I could grind back the weld.
I then went to install the inner rocker with the floor extension and found I had made a mistake and cut too much out of the floor, and I had to put it back on
. I measured from the wrong point and was about 1/4" out. I also discovered that the outer corner of the patch panel wasn't well formed and didn't reach the edge of the floor when the kick panel was installed, and also left a big gap to the floor. I made some relief cuts and reformed the corner so that it would meet.