If intake and head ports are slathered with oil coating, almost has to be intake/head geometry seal. Recently went thru almost the same on our six pack 512. New set of Eddy aluminum heads all is well. My issue was severe plug fouling,,a bit different from your problem,,but same idea,,,too much oil in combustion chamber.

Machine shop that set up new heads with bee hive springs, etc. I had check for me the valve guides. They said they and seals were fine.

No more plug fouling. Intake ports are dry,,,had to pull intake again as one of my recently installed Scorpion hydraulic roller lifters decided to take a dump. .25 valve gap on exhaust number 6.

Does it ever end!