I believe IHRA mirrors NHRA for the most part. There are a few exceptions. IHRA permits pre-1960 cars, while NHRA is 1960 and newer. Some HP ratings differ betwen N and I. IHRA may also accept cars that NHRA does not (not sure if this is the case). If it were me, I'd address a specific question to an IHRA tech person.
http://www.ihra.com/competition/index.php#contact