If for example the car came with a T-5, the OD is considered another forward gear. So it is legal to run either a 4 speed or 5 speed trans. I run a 5 speed GF5R in my Mustang. If the car only came with a 4 speed then that's all that you can use.
This may not be updated yet, but a fellow racer's combo only came with a 3 speed stick, and NHRA is reviewing this to allow what has been done for the auto cars that were only 2 speed are now allowed to run a 3 speed.
http://classracer.com/classforum/sho...ighlight=ghost
Sean