I am just now learning to deal with rust on sheetmetal, but I chipped a heck of a lot of it off in my 8 years in the Navy. We chipped the loose rust off with a chipping gun then painted over it with some stuff called "vactan". It looks like the same "rust converter" Wally-world sells. After it turned black (cured I guess) we chipped it back off to bare metal then painted the whole thing with epoxy poly amide paint.
But since I am no longer in the business of restoring the bilges of a submarine I am gonna have to come up with something different for my truck.
My question is why can't you galvanize the body parts prior to priming and painting? If you can get a 20' beam dip galvanized you ought to be able to dip your frame.