My 70 6 pack was fouling plugs black. Went thru the whole fuel/carb thing. Finally decided was an oil issue. Long tale of woe. Finally replaced heads with Eddy Aluminums. Instant cure for poor seal between intake and old heads. Heads had at one time been cut, but not intake to match.

If you give up on a possible fuel issue, consider pulling intake to see if ports are slathered in oil.

Any misfiring?

Have you considered running a heat range or 2 hotter plug. Could be running black because of a very moderate driving style. Take car out and blow it out hard on freeway, how do they look now?

Initial timing and other issues could be causing center carb running in transition zone at idle and at very low speed. Possible too high power valve number.

Possible PCV valve issue.

Are all plus black or only some.