I went back to the garage and made a supreme effort and got the rear seat unhooked (my fingers, arms and shoulders are going to be pretty sore tomorrow). Removing the seat revealed that the back is indeed screwed down, and it was now no problem getting that out. Also, there was at least 5 pounds of sand under the bench, left over from 1987 when I had the trunk sandblasted. (Yes, I've been working on this car for 20 years!) Since sand holds moisture, I'm surprised it hadn't caused the floor to rot out. But the car has been a garage queen for those 20 years, so it hasn't been rained on.
Before I reinstall the bench, I'm going to take maybe 1/16" off of the hook that holds it on, to make it much easier to get out next time.
I decided to take the rear seat out in the first place so I can run the wire for the new electric fuel pump.