It's almost a must to buy the spec. from SFI. That said, you REALLY want to get advice from an NHRA chassis inspector also.

I just had one out to look at my car on Sunday. We spent 3 hours going over everything on my car to be certain it will be right the first time. I'm upgrading from 8.50 to 7.50 with the SFI 25.4 book that's for door cars with a full tube chassis, under 3600 lbs. He had the same SFI spec. book that I have PLUS a lot more from NHRA.

Of course, if you don't have a tube chassis in the car already that makes things a little simpler. Still, some things are open to interpretation so it's not worth the risk if you haven't built a few of these cars.

It's like he told me, NHRA makes some aspects a little vague intentionally so there's a little wiggle room for different car variations.