I did that to my Camaro. I recommend rivnuts and sheet metal screws. You can make is structural if you want too.
Edit: I had a second thought. I'm not sure where the sender and lines are on the 3rd gen, but on an LT1 4th gen it sits from midway to the front. On the LS Juan, it starts farther back. If you think you might do an LS plastic tank swap, cut it out front to back and not the bare minimum. Did I mention the plastic tank swap is the bees f'in knees? With a steel tank, my car would pop and fart mid corner just around a half a tank. With the plastic tank, it goes to about 1/8 of a tank. An absolute WORLD of difference. It's lighter and holds an extra gallon, so likely a wash in weight at the end but oh so worth it for being able to run it lower especially if you're trying to shift around weight and ballast.