1. Unless you are trying to manually shift the automatic, you will never hit the rev limiter in Drive.
2. A window switch is much more reliable than "knowing when to let off the gas"
3. Not only do you need colder plugs, you need to retard ignition timing, and make sure the plugs are gapped to .030" for nitrous use.
4. An a/f gauge does nothing but have some cool flashy lights that move. It is not accurate at all, and if for some reason it went to lean while you were spraying, its already too late.