Hyde -
I went to a great deal of trouble to hide the thickness of the fiberglass body in this area. Using a die-grinder and various "tootsie" rolls, I ground and tapered this area from the inside of the body for about an inch around the opening.
I actually glued a piece of thin aluminum to the inside of the fender so the backside of the fender appears to be aluminum while looking into the engine compartment. I also glued some foil insulating material to the inside of the fender near the exhaust cut-outs, to insulate the paint finish from the heat of the headers where they pass through the body (I've seen a lot of cars with blistered paint in this area).
regards,
Jeff Burgy