any tips on cleaning out the cowl?
Ouch! I would have removed the heater box and left air box and reached up through the hole and scooped/blown/rinsed/vacuumed that stuff out. I've seen a lot of similar stuff here with all the pine needles and mold - turning to mulch.
There are caps on each side of the cowl in the hood hinge area where they drain make sure they are not packed with debris.
on my 66 I had to replace the cowl and if trying(if you can) to reach up from inside to remove stuff then you have a bigger problem than leaves. here's a picture of my 66 with the top cowl off.
there is less than 3 inches clearance after the top is on and I darn sure can't bend my old brittle fingers that much. A bent copper airline hooked to the blower nozzzle just blows it around and a vacuum is just not gonna fit in the top hat cowl area. wet stuff does not move real easy, dry does but it does not come out the darn holes easy at all. the slimline nozzle on the vacuum does fit in the oval holes but can't get the lower corner stuff behind the top hats.
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old inner cowl vs new inner cowl lots of spot welds $#%#
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new outer cowl with clean out plug holes.
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I pulled a ton of garbage out of the cowl areas before I took this picture. I had a problem drilling out all the spotwelds and it caught the junk in there on fire,causing a major panic attack on my part. The junk was still smoldering when I finally did get the top cowl off 2 day later.
yep there was a minor water leak:D
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there used to be a small opening on the side of the top cowl near the upturn part of it that would drain out water into the fender/door opening area (I couldn't find it on the old one but it was on the replacement parts from another car that wasn't rusted as bad).
if I had a good cowl(and wasn't going for factory correct looks) and was sure the top hats were sound,then I'd cheat and drill out holes like in bigblockmopars post and then just plug it with plastic plugs.