Milled heads? what spark plugs?

Another thing is just because you have a retracted tip plug doesn't mean that the plug is colder as it's just the difference of the tip location. Most high HP engines run cold plugs due to the compression ratio and the high output of the ignition systems and exotic fuels. In your case you may need to drop down a heat range or two, but I would try what you have first and go from there, just adjust the timing for the compression and look at the plugs after you drive it a bit.