if its horrible and youre missing most of the rockers and all like i am on my 87 xtra then you might as well forget it, these things rust form the inside out, i dont think they painted the inside of cab parts at all, or not very much anyways, same with GM and ford, etc,.
im not trying anything with mine, its too far gone, hugr rust chunks are packed in the rockers and so forth, and fall out on the road all the time hehe
In my humble, modest and amateur opinion there is only one thing to do with rust, get rid of it. Itll save time and moeny in the long run. Don't convert it or stabilize it, cut it out and replace it. Depends on your ingenuity.
A good friend of yours should be your local metal fab shop. My rockers were gone up front behind the rear wheel wheel, and in places around through to the quarter panels. I took it out to the fab shop, they took a thin piece of metal, wrapped it around from the face of the door trim, all the way to the top inside of the quarter panel. They cut out a 6' long peice, 2 of them actually, I cut the old ones out, scarified the remainder with agrinding wheel, spot riveted the new piece on to tack it, then welded it front to back. They charge dme $60 for both pieces, pretty minor costs beyond that, bottom of the fenders in this area were rotted out, fibreglass cloth from the good fender down around the new rocker with fibreglass resin, and done deal, primed and painted a strip along the bottom 10" or so. It's low enough that no one would notice unless I pointed it out. It's been 2 years now and it's all still holding strong.
The only tip is you really need to apply weld, with a mig or such, rivets generally shake free given enough time.
I've got to do the rear bottom of the box behind the rear wheel well, was going to cut the area out, rough out the shape in styrofoam, cut out a peice of metal and beat it into shape (a 4'x4' peice of 20lb body metal is $18 at napa), and then put it in same as above. Takes very little given a tiny bit of thought. Even if you can't weld it, rivet it and take it to someone to weld, quick job as long as all paint has been removed from the area, weld will not stick when any paint or primer is present.
Hope that helps stimulate some action!