you'll be fighting a losing battle if you use the truck regularly in the wet and with a wide range of temperatures, these truck weren't built for longevity, a workin mans truck meant to be used hard and put away wet, they were cheap consumer goods, which is why you don't find good ones on every street .... the routine maint indicated in the shop manual is a good idea, but for rust problems ....
aside from the total lack of rustproofing like is done with modern vehicles, the TF panels have one real shortcoming, the double panel of the lower body - the metal has no finish on the inside of that space, and the changes in ambient temperature and humidity results in condensation on the unpainted sides - that condensation in the hidden areas causes surface rust that over time flakes the iron oxide particles to collect in the bottom where they build up and hold moisture, eventually rotting the skins from the inside out ... just about any TF panel you'll find will have rust in the rockers all down the side, worse in front and behind the wheel wells, as the 'little spot' you have - if you cut open that area you'll find it's a lot worse than it appears and you'll see the junk inside
same inside the doors too, but the doors have drains [which don't work very well, but should not have 'plugs'], and are somewhat accessible so can be watched and flushed out .... the other bad space is the cowl vent you mention, but cleaning it out at the top doesn't do much, you need to make sure the drains at the bottom of the cowl [behind the wheel, about where the lower fender bolt is] stays open, maybe even modify it to be a bigger opening, and anytime you wash the truck, flush water down the top vent and make sure it's running freely out the bottom - and don't park under evergreen trees, the needles don't 'flush' well
but all that said, the only way to ward off corrosion longer is to completely disassemble the truck and put a good rust proofing on
all sides of every panel, then keep it exceptionally clean and at the most constant temperature you can - eg: in a heated garage
other than that, use it, keep it clean,
restore it again when neededBill
