dark plug on cylinder 6

Too much resistance in #6 plug wire ? Or poor fuel distribution of intake manifold.Kinda common for the slant.Try this,, drive the car at speed(50 - 60) for about 10 miles pull over and shut it off quickly. Pull that plug and see what it looks like then.Several slant guys that I was racing with actually ran hotter plugs in the #1 & #6 cylinders,me included. I believe the 71 Slant called for N-14-Y Champion number. As more smog equipment was added the plugs went to RN-12-Y(or later YC).Just a thought.