1968 Formula s 340 Here we go again
My son has a car that rodents packed the frame rails with debri making a nest had few pins holes in frame rails. We cleaned rails out via holes drilled into floor from inside car, cleaned debri out, sprayed Eastwood stuff inside rails, then put body plugs in floor to close the holes we drilled. Car still has pin holes in frame rails but we stopped the rusting process.
I have had good luck removing loose rust scale and brushing on rust converter to stop rust.