You absolutlely have a traction problem,that one leger aint gonna cut it.If your spinning though first gear that is the first think to fix. (Read much better gears/posi/good drag tires) Remember that the 1/4 mile is a specific race and such the chassis needs to be tuned for it. Seems to me that although the motor goes flat at 6700 RPM, with your gears your not through 4th gear., are you? Think of the 1/4 mile race as an 1/8 mile, but the second 1/8 your just pushing the loud pedal, not really but think this way. I would not run any laughing gas untill you can launch very hard and get to the top end of 4th gear before the end of the race.
paul