I could see PORing the floorboards and the inside of the bedsides where the rust typically starts, but you won't be able to get at the inside of the cab, rockers or frame. If nothing else, our trucks are very reliable with where they start rotting. I would hit the important spots, the underside and most of the frame, and then just paint the exterior like normal. The POR won't really be preventing anything there because that's not where it typically starts (unless the paint has been damaged). Be sure not to plug any of the drain holes, or you could do more harm than good.
Since my 88 is already rotted beyond reasonable repair, I bought myself a backup truck (the gutted but rust free 89 hanging out at my parents' place in Georgia) :D