Just found this on the MSD forum:
Actually, we recommend that when using an MSD CD ignition that the user stay away from any of the speciality or 'hype' plugs. The reason for this is that these type of plugs are a hotter burning plug. Combined with the hotter ignition of the MSD CD; there is not enough time for the plug to cool off between firings. Eventually, the plug could start to glow like a 'glow plug' in a diesel engine. As your air/fuel mixture is compressed in the cylinder and getting ready to fire, the heat from the plug could cause the air/fuel to ignite permaturely causing detonation problems.
We recommend using a plain copper plug; 1-2 steps colder than factory recommendation and increase the gap on the plug in .005 increaments until you have a nice smooth running engine and there is a nice tan/carmel color on the porcelain. This could end up to be in the .050-.060 range.
MSD Tech Line: 915-855-7123
so standard NGK and maybe a grade colder or so. Interesting.
So no iridium basically.