I'd go with POR-15. Some people talk trash about it but I think most failures of POR-15 come down to not following the directions. They even have a very simple "cheat sheet" that they call "directions for people that don't like to read directions"
I had similar surface rust on my 3500HD project; I knocked all the loose stuff out with a wire cup brush on my drill, then painted almost the entire cab interior. I did the engine side of the firewall too, as it was formerly a diesel and the insulation pad that was there had caused a little surface rust. Plus, the truck was getting painted black afterwards...
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Just please...please...read the instructions carefully before you use it. It's nasty stuff. But, not nasty like bedliner!
Richard