So I have the Motorcraft F11 plugs always seemed to have a couple that wanted to leak, well I just found that one plug was made with a bad thread, last two at the top are really messed up. So in pulling the plugs several different times, setting timing and so, the bad plug would find it's way into a different cylinder. I ran a new 1/2 in. tap through all the holes and no damage I can tell of, have ordered a new plug and ask Lang's if they had heard of a problem and it could have been a few years ago. I did not buy these plugs, they were probably new when the motor was rebuilt by seller. OK, this should help, still learning
Your cylinder head is decades old with unknown times plugs have been tightened too deep, rust, wear, and the old iron threads just seem to get poor. New plug threads sometimes are different forms too, so leakage happens.
For me the best fix is to cut a small length of alum foil from the wife's kitchen cupboard. Wrap that foil around the spark plug threads that engage the head in the direction of seating. Wipe some anti-seize into the head plug holes. Insert the wrapped plug and tighten down the plug to snug.
No more weeping, seeping.
All times, with new plugs in known good cylinder head, like new alum hi-comp heads, a dab of anti-seize on the threads before placing the plugs. Seals and also keeps threads good, and eases removal.
I hope you didn't run a tap all the way through, the head thread is a tapered pipe thread to match that of the taper of the spark plug, if over taped the plug will leak or even blow out, if threads are good then use some aluminium foil around the plug thread.
I have no personal experience with this, but read an old thread the other day on this forum and Teflon tape was a common fix. I would think that the heat would be an issue, but I did remove some champion x plugs from an engine I was working on that had Teflon tape on them.
White teflon plumbing tape is what I have been using for years with no issues.
I have a aluminum Z head on my 1919 Touring and rather than worrying about messing up the threads in the future I installed adapters and now use more modern metric plugs. When I have the occasion to take out the plugs the adapters stay in the head thus preserving the original threads. I know this is not as original but it works for me.