for fiberglass tunnels I cut the floor back for clearance and then put a piece of the foam rubber packing that the Pro Chargers superchargers come packed in--it is smooth on one side and kinda egg crate on the other---this will form about 1 1/4 from the trans--put waxed freezer paper over the top of that and start laying up fiberglass cloth and mat---can also use kevlar/carbonfiber if you want---end result, you have a floorpan with clearance that you can bond down as you build it or you can overlap the floor edges and have a screw down that fits perfectly with a consistant 1 1/4 clearance