In order of effectiveness (and probably in reverse order of ease):
1. Cut out, weld in new metal
2. Sandblast - it'll get 100% clean, but prepare to be amazed at how much rust you can't see (almost all of your spots will be thru-holes when done)
3. Naval Jelly in repeated applications to dissolve the rust, followed by a converter (Loc-Tite, etc) to convert any remaining (non-visible) corrosion, then topcoat of your choice.
It's all in how far you want to go and how permanent you want to make the repair. Honestly, it's in a somewhat hidden location (wouldn't have to fully repaint the truck) with easy access and flat, or very simply bent, pieces. Personally, I'd cut it out and weld in new metal coupons. It's the best repair and would give me the piece of mind of knowing what's behind it. More than likely, the rust is also on the backside - at least in the first 2 pics. Probably over the top for most people, especially those above the rust belt that deal with way worse, but it's what I'd do.