Looking at a used (well used but never been used) MSD 3 stage multi retard (all silver box) that my boss has that is giving me a good deal on it.
You have to wire it into a 6 or 7 series box (not a problem). Since I will only need the one stage for nitrous can you wire that to come on by wiring it into the nitrous arm switch???? Since I will only need it when spraying?

 

Its better to just have it activate when the solinoids come on. Performance will suffer and the car will idle poorly if its retarding off the nitrous.