If the grade is too cold, the tip wouldn't provide sufficient spark voltage to ignite the mixture, hence run rich etc. If your running 400bhp on cold plugs, I would guess and say you've switched from the dizzy setup to atleast wasted spark where the coil has a longer dwell period providing greater spark voltage, so even at cruise there will be plenty of voltage to ignite the mixture. You need to go colder because the extra fuel injected creates more heat and increases cylinder pressures, running a grade 6 plug at 400bhp will most likely drop the tip off the plug and make a mess of your pistons and cylinders. CDI ignition doesn't have such problems, but expect to be changing your plugs every 3000 miles as they don't last long.