As you'll recall the rear wheel wells were really ugly. They were flared and consisted of sheet metal, bondo and a few dozen rivets.
The wheel wells were cleaned, wire brushed and ground down to bare fiberglass. The replacement inserts were trimmed in order to preserve as much of the original body fiberglass as possible. Fiberglass tabs were made and riveted to the backside of the wheel wells. The outside of the wheel wells were then feathered back with a grinder and sheets of fiberglass mat were layed down.