...it doesn't say you cannot. It does state .....full upholstery must be retained, equivalent to factory specs. Since this is not only the way the car was built, but raced successfully, NHRA certainly deemed the removable steel tunnel equivalent to factory specs
if the seats are 5 lbs lighter than "factory" seats....I would be surprised. The weight savings are in the (legal) aluminum seat brackets.
I have read the rule book from front to back and so far....as built, I have only found ONE thing that coulda /woulda possibly been deemed illegal.....and that depends on the meaning of what "is" is. Would not be considered a performance enhancer by any stretch. ......Butch Leal and the 396 cu in Hemi right out of his 73 Pro Stocker (both legal)....how much more trick do ya need?