So, I got to cutting the spot-welds. I'll have to remove the passenger seat support, which is probably the toughest part of the job. Besides fitting the panel, that is. I have a picture here of the hole getting larger in the floor--cuting a little bit out at a time is what's needed here. Baby steps.
I also have a picture of the removed inside seat belt support, more as an example of how cutting out the spot welds works. The spot-weld cutter is more or less a tiny hole saw. The trick is to not cut a hole through both panels.
It sucks having to cut perfectly good metal out of the car, but in this case I think it will be better in the end.