Long before this was a hobby, I used a rust convertor on my floor boards after some wire wheel treatment and then fiberglass. That was 38 years ago and 200,000 miles later and 15 years of daily driving after that and I still have the same floor boards with no evidence that rust has come back. I was a poor high school student at the time and did what I could. Never thought I would own the car this long or that I would not have to touch the floor boards again.
So...what ever they had back then was good stuff. I would imagine that you could easily do the same thing.