These are the rules from the current rule book:
25. FIBERGLASS COMPONENTS: All fiberglass components must remain within the stock dimensions, thickness, location and configurations as currently delivered by U.S. Legend Cars International. All fiberglass components must have an authentic INEX certificate embedded into the underside of the fiberglass component, evidencing that the component is a certified US Legend Cars International part. Fiberglass components may not be reinforced or intentionally lightened in any way. The use of a complete front end (Matching hood, fenders, grill and grill shell) on a different model car is acceptable. It must be complete with all listed components from the same model. Mounting of Components -All fiberglass body components must be firmly attached to every Legend Car competing in any race. It is recommended that all dzus fasteners fit tight and are taped over to prevent loosening. Any Legends Car may be black flagged or denied entry to start a race if any of the aforementioned items are not attached unless prior approval is given by the INEX technical inspector. Removal of undamaged fiberglass components (hood, deck lid, etc.) during an event is not allowed. Rear of body may be trimmed between the frame rails up to the top edge of the rear frame horns.
26. FIREWALL: A metal firewall is mandatory. Firewall must be installed as currently delivered by U.S. Legend Cars International. Using a “thicker than stock” metal firewall separating the driver’s compartment from the engine compartment is OK.
Not sure what that all means, as they do not specify minimum/ Maximum thicknesses.
Steve