It's going to be engine- and vehicle-dependent. Different compression ratios, different fuel delivery systems with different emergent issues from endurance racing, different cooling system efficiencies... but it sounds like you already got the advice you need regarding a stock 4.6 mod motor. Don't fuck with it.
Just for the reference of those running very old engines, we run one heat range colder than stock on a 60s Chrysler slant six (NGK 5 vs. 4). I've read that the original plugs had to be hotter to prevent fouling when burning the lower quality fuel of the day. Ours are a beautiful light brown whenever we bother to check them. (We also have a slightly higher compression ratio than stock, so I can't directly confirm.) Might consider going even colder if we ever get around to adding forced induction.
One other point for those driving slant-sixen or other 60s tech. Some people like to run extended-tip plugs (eg. NGK ZFR5N) on older street cars that didn't originally call for them, because it supposedly improves combustion efficiency. These are not generally a good idea for racing as the extended tip can be a hot spot and contribute to pinging under heavy load. YMMV
Command Pilot/Flight Director, Escape Velocity Racing
Index of Effluency, Heroic Fix, Class C Trophy, now hopelessly gunning for Class B.
Currently 1-2 vs. Team Fairlylame in the Class of 1964 Championships