One thing about those V6's is that a lot of junk falls into the spark plug holes with them being on top of the motor. Glad you found your problem was an easy fix.
I had never thought about it so I also was surprised plug threads were metric. I did some research:
Spark plugs were NPT threads on Model T's, but in the 1930's manufacturers started using metric (14MM and 18MM) threads. Evidently Ford switched from NPT to metric on flathead motors in the 30's. So early flatheads are different from most of them.
From what I have heard it had something to do with Albert Champion moving from France to Flint, Michigan to start Champion spark plugs and AC spark plug companies. Being european, he would use metric threads.
http://www.smokstak.com/forum/showthread.php?p=350782 Also I found that the best way to deal with this is not with a tap but with a thread chaser, Summit Racing, et cetera, sell specific tools for this with 14mm on one end and 18mm on the other.
(Now when you get to screw-on oil filters, some are metric threaded and some are SAE standard threaded, and--don't assume all Japanese stuff is metric. Mazda's are metric and Toyotas and Kawasaki air cooled motors are standard. Go figure...)
Did you do the rebuild on your motor? My son has a running motor with about 95K original miles, and found a spare that we are planning to rebuild and it seems pistons and main bearings are hard to find. Those things are made like anvils. My engine stand groaned when we loaded that big boy on it.
