If that were my truck, I would do the following:

Use a pneumatic needle scaler and go to town on the frame. Wire wheel (minimize the wire wheel as it'll burnish the rust which you don't want) and sand the area's that need further attention.
Then get some Corroseal (it's a much better alternative to ospho as it's easier to work with, won't ruin everything it touches) and corroseal the ENTIRE frame, literally everywhere you can brush it on. Make sure to follow the application guide well, most people don't use enough coats and it's too thin. More is better than less.
Then finally I'd use a good paint to protect it from further corrosion. I've moved away from Por15 and use Rust Seal myself.

I live on the gulf coast and things rust, BAD when left outside and not used. staying ahead of the rust is key. I've found the simplest way to do this is to liberally apply a product like Fluid Film to the chassis twice a year. You can use 3M Cavity Wax for inside the body panels.
The previous poster who mentioned the drain holes is correct, you've got to make sure your frame drains properly and doesn't store moisture.