I just made the switch to NGK's from Accel 416's. I contacted their tech line by email and got excellent advice from their tech guy.
I make the same power as you with a 427-sized FE with a strong cam and suffered the same sooty plug problem. The tech suggested their #4 heat range, platinum tip, fine wire plug in the G-Power series. It is NGK #2685, FR4.
Here:
http://www.ngksparkplugs.com/product...r.asp?mode=nml
I have a 6AL so use a .045" gap. Presto-plugs I can actually see the insulator tan/white and off idle performance is noticeably cleaner. Shifts at 6600 are clean with continuous power. They have perfect manners in street use. A hotter plug is a good thing.
Be sure your heads will accept a .750" reach, projector nose, resistor plug. They cost $3.39 ea. at Summit.