If restoring the truck, unfortunately that frame pretty much has to go. You could get away with patching or at least reinforcing some of the weakest areas, but it all looks pretty thin from what I can see, though I'm sure some parts are not nearly as bad as others. Especially when it gets "flaky" looking like that, those parts are just pure rust scale, all the grinding in the world wouldn't turn up shiny metal.
With that being said, that frame is by far not the worst I've seen (or driven around with). If you're currently driving the truck it's not like you have to park it immediately and find another frame ASAP, that one will hold together a while longer. It's got the cancer for sure, but it's not on its deathbed yet. I'm not saying "don't worry about it, the thing is fine" because it's not, but I'm also saying it could be much much worse. If you were to put the bed back on it and drive around once in a while it wouldn't just break in half tomorrow, but I would personally start looking for another frame eventually if I was serious about keeping that truck on the road. For now keep it clean, maybe oil it, don't take it over any jumps lol. You still got a little time to find a new frame.