I made some progress this weekend!
I borrowed some steel boat stands to get the car up nice and high to work underneath:
Of course, the more I worked, the more bad metal I found. (The latest is a hole behind the plastic fender liner.)
But the floor itself is now mostly done, aside from cleaning it up, sealing, painting & undercoating.
I'm not going to win any welding awards. But it'll do the job.