I have the block off plate for my side cowl vent. Even With the plate and a new gasket on, I would imagine that water will still get in there in the rain or while washing. I see the hole at the bottom of the block off plate to drain water but doesn't that just ask for rust inside

 

Of all the places my truck was rusted out back before I had it fixed and painted in 1999, that area was not one of them. With the louvers beveled as they are it is difficult for water to get in there unless you squirt the water up and in the louvers. I got a block off plate in a junkyard once and it was in great shape with little rust.

 

That's how ford designed them. There are drain holes at the bottom of the cowl so the water will drain. One of the members here (can't remember who) modified their plate and attached a drain tube to it and ran the tube out the bottom of the cowl.

 

All corners of your cab should have drain holes to the outside. The problem with these old trucks was they were used as trucks, i.e. farming, ranching, construction etc... This caused dirt and junk to plug the holes which led to moisture retention and rust out.

If the drain holes are clear there is no issue with the little water that would get past the louvers.

 

The other issue is that windshields and backlights often leaked when these trucks were designed. So they designed the cabs so that water could exit. My replacement cab is in much better shape and I see that there are even drain tabs in the rear half of the floor in addition to those that you noted at the bottom of the cowl. Even with this better cab one of those cowl drains is rusted out and will need patching.

The problem is, as Dave noted, junk gets in there and clogs the drain holes (my first cab those drains were completely rusted away and the rear cab corners had rust holes) and worse yet in the winter in most of the country they freeze as not many of these old truck were parked in heated garages.

Since, like many here on FTE, I will be installing sound deadener and carpeting I'm going to fill those cavities in the front and the rear cab corners with intercellular foam that does not absorb water. With the new sealers windshields and backlights can be sealed so they do not leak.