On my 58 gmc steps side there are no inspection plates for the cowl and vent areas from the fender side of things.
I did however remove at least a full gal by volume of sticks, leaves, and assorted debris from each side of mine when I got around to that area. At first I didn't realize that small triangular area was even a weep hole. I cleaned it out with a screw driver and opened it up a bit thinking it was just mud in a crevasse. A day later I was knocking around and more dirt started falling out. So I poked and prodded a bit more. Finally I realized it was one continuous duct space all the way down the side of the cab to that 'still' plugged up hole. After a good 30 minutes of vacuum at the bottom air nozzle down the top via the cowl and a rubber mallet banging on the side I reached a point where the dirt and dust stopped falling out at the bottom and the rattling of leaves and sticks inside the vent area ended. I will say it all looked like grade A composted dirt and I was wondering just how long it had been composting there considering the truck had sat for 18 years before I got a hold of it but that's for another forum I think.

This thread has made me consider making an inspection plate in that area though. It would be very easy to do and make future cleaning of that area a LOT easier. Sort of like reversing the bottom two bolts for the timing cover so you don't have to remove the oil pan to loosen them to pull the cam. I don't remember where I read that but it sure was a great tip.

Give me ambiguity
or give me something else