Milled heads? what spark plugs?

63dartman,
The plugs only change by heat range and either a extended tip or a retracted tip. Generally milling the heads will only change the amount of timing that the engine can run and or make you run higher octane fuels due to compression ratio changing. As for the actual depth of the plug changing, it should be in the same place as the plug sealing washer is sitting in the same place and can only go so far. Milling the heads doesnt change the plug depth. If the tip is hitting the dome of the piston then a retracted tip plug is in order.

Most plug applications books have the differences in the back of the catalogs and a cross reference for changing tip locations and keeping the same heat range. Go to a parts store that still has catalogs and look in them and get the plug that you want or need.