There should be a thick black weather strip sitting flat on the leading metal edge of the cowl. That seals against the bottom of the hood. I removed mine briefly, thinking it would run cooler at the track by letting the hot air out. But when it rained, water poured in, particularly onto the PCM. If you need a photo I can post on later when I'm on my PC. You can see a small version in the pic on the right side of my signature.
Don't know the part #, and don't know if GM still sells it. There is a parts manual somewhere on this site.... can't remember where. And online auto parts vendors like Nally have detailed parts listing that graphically show the part, part number, whether its available in OEM and/or aftermarket.
You can see it in this photo. It attaches with plastic push pins through the lip at the rear of the strip.