I am not well versed on NHRA or IHRA rules. But I'll pass along what the SCCA allows. I know this may be apples and oranges, but it may point you in the right or similar direction.

In the SCCA, it's not uncommon to have co-drivers at certain events. We are allowed adjustable seats. The rails are bolted to the floor and count as being permenantly attached to the floor. The SCCA does require a seat back brace that bolts to the seat. The reinforcement can be adjustable where it attaches to the cage.

Now the big difference here is what's required in cage construction. Minimal cage requirements in a sedan is a bolt-in 6 point cage with one bar for side protection. Most cars will run 8 point cages with NASCAR style door beams.

Again, take it for what it's worth. Different sanctioning bodies and different issues regarding driver safety. SCCA classes can include as many as 60 cars on a given track at a time, but at various speeds from 40-60 MPH on some turns to over 160 for faster cars on high speed straights.