Frame and Floor repair -What to do?

[daedal.]

I've got a '91 Chevy S-10 pickup that I'm currently re-building and I've decided to start with the frame and build from there.

Now, this raises several questions.
1. How do I fix rust on a frame? Simple Grinding? The rust seems to be simple corrosion without having actually eating through the metal.
2. Is there a filler material that I can use on frames that is legal? I don't think applying Putty on frames would make it safe (or legal), now would it?
3. Once it IS repaired, what can I shoot/coat it with to prevent further rust?

Once that's done, I want to tackle the floor. I know how to fix it but I'm not sure what I coat it with on the inside of the cab? Just regular Primer and Paint or is there something thicker to use to prevent rust? Because the carpet will be going on this thing, I can't use something too thick either so I was wondering what to do.

Thanks!