nos is a brand name of nitrous that Holley makes. What you are looking for is N20, or nitrous oxide.
the 91 octane gas, was that using ethanol? If so, you dont need to retard the timing as much since the flame speed is higher with alcohol. Also, these engines have the timing pretty far retarded from where they can run anyways, so you can actually do quite a bit without needing a higher octane or needing to retard timing. Plus the nitrous really cools the intake charge, so you reduce the need for timing and octane slightly. Although I would be concerned with a "75 shot" at 15* with 91 octane.
at the 100 octane, you should be able to stay ~12* timing safely with the "100 shot", though putting that much nitrous in these motors..... I would really recommend a visit to a dyno to tune, especially since I would be worried about leaning out even with the 75 shot. I hope you have a wideband, I would want to keep it fairly rich.
edit: never noticed you were using a dry kit till I read the other posts. --fail