Here are some pages showing what it looks like in there:
http://67-72chevytrucks.com/vboard/s...98&postcount=7
http://67-72chevytrucks.com/vboard/s...9&postcount=15
I like the shopvac idea. You can see how the metal beneath the vent slopes to the rear. That is where the 'drain' is. I'm not sure if you can get to the 'drain' with the fender installed, but you could almost certainly blast it with some compressed air while the shopvac is running inside the vent.
That 'drain' is the only exit for anything that gets inside the cowl. Can collect leaves and pine needles that will hold water and rust it out down there.