I got the first floor panel out. I had to cut it into three pieces, but it wasn’t that difficult. 
And I had a realization. I’ve been thinking of cleaning the rust off the frame and painting while I’m on a creeper underneath. How silly! It’s about 100 times easier to do it now, while the floor is open. The rust isn’t as bad as I expected.
Definitely rusty, but only fairly light surface rust. That must be because (according to the previous owner, who I believe was the second) it was parked and not moved. So I cleaned the rust off. Mostly I used a 4.5 inch grinder with a wire brush, but I have a small sandblaster and used that where the power brush didn’t reach well. At this point I looked at the welds, and all look good. Decent quality and no visible cracks. And the first coat of Alumathane is on.
Tomorrow I’ll get the second coat on. Not sure if the weather will allow me to replace the floor. I might have to wait. I need an hour or so of dry weather, and tomorrow is supposed to be wet.