You do need extensions, but that's not causing that much surge, and neither is the chassis, carb, or converter. Check your WOT switch. To see if it's not staying engaged.
If you have a grid, download the run and look at the step switch for timing, I bet it's on and off.


I just read the part about a fuel pressure switch, and that would for sure do it. Just a thought though, if your primary pressure drops that much, I wonder what the nitrous fuel pressure is doing. 400 is a good size shot to have the pressure fluctuating more than a quarter of a pound.