your floorpan doesn't fall off. :roll:
This is a common misconception. If you looked at the structural integrity of any unibody car, the floor pans are more often than not checkerplate that is installed afterwards for ease of manufacturing. When you make a lot of power with something like a turbo, the whole unibody takes the load and it slightly twists. This is, however, not the case when you have NOS, LOL. The big difference is that the molecules of NOS have such high momentum that they can escape the manifold and can hit the firewall at wierd angles. If it hits it at approximately 34.68 degrees, it will cause the unibody to hit it's natural frequency and shake that poor checkerplate off. This also causes the valuable green washers to drop out of your dash. Those are what cause your manifold to become volatile.
Don't let anyone tell you differently. I'm an engineer, I know how these things work and I DEFINETLY wouldn't steer anyone wrong, ever.
As a side note, I strongly recommend you rip out your carpet to expose the checkerplate underneath. Don't be afraid to just rip it up, as you won't need it anymore.