Floor Pan Opinions?

I second the "don't worry about it for now". Until you fix the rear leak that caused the rust, anything you do will just rust again. At least you have a drain now under the rear seat. In the front, don't do change it unless you are ready to do it right. Welding can cause other problems like weld splatter can ruin would windows or dash, or even set the car on fire. I would touch it unless a total restore.

Some people over worry. Do motorcycles have floor pans? Will a few holes threaten your life? Actually they can if they allow exhaust fumes in, your exhaust pipes leak, and your engine runs rich.