I used exterior 1/2" ply with two layers of biaxial on each side... Rock solid. It ended up being about 13/16"- 1", a little thinner in the center, a little thicker at the edges where it's bonded and laminated to the body, zero flex, or at least not perceivable. My body is fairly heavy compared to what I started with, but it's very strong, doesn't flex much, has steel supports, etc... I think it's at least a couple hundred pounds, I can lift it, but it's cumbersome to install and remove from the frame without help. I feel it will be more safe, comfortable, and the added weight is minimal in the big picture. Basically a passenger. Jmo I seriously doubt the 427 will notice. It's no longer a drag car, lol. I think the added weight will actually benefit me because it's centered, and will help the springs function and improve the ride and handling quality.