Couldn’t find anything on the thickness of the transmission cover so I’d suspect a minimum of .032” aluminum or .024” steel. You can always call your NHRA division office and they can probably point you to a tech person that could answer that question.
The rulebook:
Section 4A, #7
“Subflooring independent of car body is mandatory in dragster that allows driver’s legs to rest on belly pan or chassis.”
Section 21, #6.1
“All interior panels (firewalls, floors, wheel tubs, doors, etc.) within the driver compartment of enclosed cockpit cars where the driver is located behind the engine must be constructed of materials other than magnesium.”
Section 21, #7.5
“Belly pans and subfloors enclosing engine or driver compartment must contain suitable drain holes so that liquids and foreign matter cannot collect, thus creating a fire hazard. Minimum .032-inch aluminum or .024-inch steel.”