You need to find a way to retard your timing when you spray or that will be the last run that engine is likely to make. It sucks that MSD can't read your ignition unit because they make one that comes on automatically when you spray, but they aren't the only ones who do.
I have an ignition system called a BRT-Tis . I have a switch on the bar. I can select between two maps on the fly. When I make an all motor run I have close to 43 deg of timing across most of the RPM band.
When I spray nitrous I will start with 35 deg across the entire RPM range ( stock ignition curve ) and work from there.