Rather than slow the car down, why not get the car teched and see what you need to make it legal ? If you're a drag racer you won't be happy slowing the car down.
Find out who the NHRA division director is for your area and give him a call. The biggest majority of them are very helpful and give you guidance. Getting the NHRA licence will get you into any IHRA event also since the IHRA honors the NHRA licence, but not the other way around.