Our trucks rot from the inside-out. whatever it is that you choose to do has to be done inside the frame, inside the rockers, inside the bed crossbars, and up inside the bedsides above the wheelwells. and be sure that anything you apply isn't trapping the salt in a new place. salt and moisture will get everywhere. It's best to just plan for that rather than trying to keep it out.
Honestly, the rest of the salt spray can be washed off in a car wash. it's the places you can't wash that are the real trouble spots.