do you know if any of the spark plug holes have been repaired or rethreded? that sounds like it would work with the marker and scratching it.

if it does or does not,pick up 1 autolite 65 plug and try it.

does the heads have air injection holes drilled in the heads?

are the holes pluged if so?

header gaskets/flanges will not seal those off and are a exhaust leak that will have you going nuts looking for it. some of the j heads have the hump in the port and mached area at the flange but are not drilled. ( I chased a rocker/lifter tick for a long time till I found the one injection hole open.