This will be a good opportunity for you to practice your welding.
(I had been welding for a couple years at your age.)
Get some sheet iron, about 18 gauge. Take out all the bad stuff and weld the
sheet iron in its place. Your sheet metal shop may be willing to cut it to shape for you if you make cardboard templates of what you need. (This is a bit oversimplified, but that is what needs to be done.) You will need to be careful of gas lines, brake lines, cables and other things under the floor, both when you cut the rusty stuff out and when you put the new stuff in. Appropriate heat shields will be a big help.

It will make it a lot easier if you remove all the seats and seat frames while
you do this. Also be ready for a few stiff muscles. I didn't do a job like this until after I had been wearing bifocals for a few years and I can tell you that it is much harder at that stage of your life both in terms of seeing what
you are doing and also in body contortions. OTOH I was in a car less roomy than yours, a 66 122 wagon.
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George Downs Bartlesville, Heart of the USA!