I suggest, if you haven't already done so, pull the seat nd roll the carpet back. If there's a hole under the seat then most likely the rear window leaked and the water also ran down to the floor pan......and sat there and rusted. you should also check the front floor boards and toe boards for the same. Novas also like to leak at the front corners near the windshield and the water goes down to the floorboards and ....rust. figure you will be replacing at least 25% more metal than you can see. One you have a better scope of work you can figure how to go forward.
I found one hole under my rear seat and ended up replacing most of the for pans ...but only in the low spots, the metal under the seat was like new.