Get some butyl acetate strip caulk! It is MUCH easier to use than silicone, and GM used it stock to seal some of the holes anyhow. You mush it in with your fingers. It is sold by 3M and you can get it for around $20 at your local paint and body supplier, or maybe even NAPA. If you can't reach all the suitcase, you can access it by removing the fender and inner fender. It is a PITA, but should not take too long. I just removed one on a 1973 Camaro to gain access to the door bolts for alignment, and it took us maybe 30-45min.