Well, if you get this wrong, you'll have fumes in the car. So, most of us here are also forum members at www.SAACforum.com Go to the 67 Shelby section and do a seach. This comes up often.
There are 2 different sealers you use. And, the thing to make sure is that the corners that you can't see inside the tail light fiberglass panel yet out side the metal Mustang tail light panel are sealed closed.
Open of the sealers is clear silicone. You also mount/seal the side scoops with this (Concours correct). Put a bead on the scoop, hold it on the car to smusch it out. snug down the fasteners. Wipe off the excess with a wet rag.