No biggie, just tryin' to help...
In that case, you could give NGK a call... 1-877-473-6767
You'll get a message prompt... just push 2 twice and that'll take ya to spark plug tech, a gentleman by the name of John should take care of you.
Otherwise, maybe someone here can help you
I'm not totally sure but if I recall correctly... for Autolite, it's like 24, 23, 22, for iron heads. I'm new to NGK's so can't help ya there. Nitrous Express referred me their way...
seems to me your best bet would be to call John @ NGK
Take care...