a BP7ES without the P is...duh...a B7ES. The "P" stands for Projected tip. a B7ES is a much shorter firing tip than a BP7ES, what application? if the stock was not a BP7ES or other projected tip then a projected tip application would be dangerous to run. a BPR7HIX is the iridium plug to replace it with, but adds a resistor to the line. If your stock plug in NGK was not a projector tip, don't replace it with one!
Try sparkplugs.com. They have a complete crossover available. I think the ND crossover was IX22 for $11. Here is some info they had about the Autolite Iridium. It isn't a real Iridium. "Autolite has introduced an iridium-enhanced plug (the iridium content is too low to call it an iridium plug) we do not consider this in the same class as Denso, NGK or Champion.
SIDENOTE: Thus far the tech's we have spoken with report no problems using iridium plugs with Nitrous."