the reason why some say .40 gap is because then they will burn better. shorter gap causes a stronger shark hence burn the fuel more efficiently. heres the down side, they will burn out faster. the .60 gap is for max life span of the plug with still burning enough fuel for proper operation. imo if i had a hotter coil i would get to about .50, if using a stock coil then around .35-.40. here again this makes alot of difference as what to gap to depending on any and all engine mods that you have done to include exhaust work. if everything is bone stock performance wise( this does not include k&n and a chip and then your have a hot rod like so many think) then i would stick to around .55-.60. if your exhaust is opened up(meaning no cats or punched cats) and/or head work done then start looking at a lower gap.
imo, i would only use autolite plugs. my t-bird is pushing about 15.1psi boost at a flow rate of around 800cfm on a 3.8 v6 with 60lbs injectors tune to 42lbs at a 40% cycle rate and 26degree timing curve and never skips a beat.