Hmmm... these are good things to check, but an intake leak would either suck oil (oily plugs) or air (white plugs). Valve stem seal/guide would suck oil (oily plugs). Chimney soot sounds rich for sure.

Too much fuel or too little air.....EGR problem? "Maybe" lean the carb out a little bit...maybe #6/#8 are just more prone to showing a rich condition? Really, really dirty air filter? Cam not degreed & again #6/#8 are more prone to the rich issue.

I'm grasping at straws here obviously. If you've done all the ignition stuff, then the fuel delivery system is the next place to look.