Is there supposed to be a gasket on the back of the hood?
 

Because if there is, I don't have one!

My distributor keeps fouling due to being wet EVERY time it rains. Not soaked, but wet.

I'm trying to figure out why!

You don't have one because there isn't one. Some water can and does get thru there but it shouldn't cause a problem unless you have bad wires or a cracked cap.

The cap and wires are new. Every time it rains I have to park the truck because it starts popping and backfiring through the intake. If I let it sit and dry out it runs fine. Usually I find condensation buildup inside the top of the distributor cap.

a piece of door trip rubber should fit nicely on the front edge of the cowling, with the biggest part facing up. When the hood closes, it should provide some kind of a seal. I don't know if this would trap too much heat under the hood though. Maybe you can put a short piece just about a foot from one side to a foot on the other side of the distributor? That way any water that does seep in won't drip right on the cap.

Another thing you could try is some high-temp, non-conductive, non-flammable (when dry) water sealing spray, and soak everything down with it. Ask the guys with the snorkels what they use, bet ya there is something inexpensive that would work great.

Those are some good ideas, I'll see what I can do.

I coated the cap inside and out with some wire dry/sealant spray but it didn't seem to do anything.

i think you could try spraying the outside of the cap and wires with wd40 if you have any lying around. wd40 usually has some type of silicone in it.