It's a 2 valve, 1993 FI, 460. I'll look into the timecert. don't know about those.
My guess is that it's just the first few threads. I can't even start the plug. However, the plug was in there pretty tight when I removed it, so it's entirely possible there may not be much left.
I think you are right, pulling the head is the right thing to do.
If I do pull the head, can I then tap from the inside out? I haven't done this before.