Looking for advice on fixing rust on interior of car

3 options

1. Buy a good donor for the sheetmetal to replace the damaged steel, learn to cut, weld, fabricate.

2. Buy a better body, and use your car as a donor.

3. Ignore it.

PS: honestly how bad it looks in the pix, it's usually 10x farther along underneath where you cant see it until you open it up. My vote is get a different body shell that's not ate up in the roof area, and turn yours into a donor car. It's a 76, likely a slant 6 car. Unless you have some serious emotional attachment to it, it's not worth repairing.