The rocker panels are structural in my opinion on these cars. Something to keep in mind.

I would weld or get someone to weld in strong patches and do a little fill work using mar glass or short hair fiberglass. Then drill holes on the backside of the rocker panel and spray oil in there mixed with a little grease. This heated by placing a container of it in boiling or pretty hot water to thin it first. I use an electric single burner old hot plate.

For all practical purposes the rust will stop or at least slow to an almost neglishable rate from that point forward. You have to make sure you spray oil into all the cavities.

I think I remember there is a bulkhead pretty far forward on the rockers on these cars. So you have to make sure you spray your mix inside on both sides of it. Using almost anything else inside the rocker panels will probably speed up the natural rusting process.