Is there any advantage to using the 14mm modern plugs over the originals
Only if you have a head which is bored for 14 mm plugs. If you are running an original head, any of the Model T plugs will work. I have used Autolite, and Champion. Both work fine in a standard T head.
No advantage, but extra cost. You need adapters to make them fit a stock head.
The advantage I guess is that you can easily get a plug in the heat range that you need. There are many more choices in a modern plug.
The modern plugs are cheap and you can make your own adapters but the plug will sit further away from the fire.
I've tried them before but now run ones from rock auto.
There are modern plugs that are cheap and will fit without adapters, like Autolite 3095. $2.19 at Advance Auto.
I'm sure I've read where "metric" cylinder heads were made by Ford for certain export markets. And by metric they meant having I believe 18 mm spark plug threads. So much for the term "modern ". The bolts were still SAE and UNC
I run 14mm plugs in adapters in alloy heads. The adapters stay in place and the plugs are swapped by screwing them out of the adapter. I like to think I am doing my best to preserve the threads in the head. It's a bit like having a brass union in a pot metal carburetor. The union takes the work rather than the soft pot metal carb body.
Allan from down under.
Yes, Les, I've never seen such a head here in Sweden, instead I think they were sold in France and other continental european countries in the early years when it was hard to find plugs with 1/2" pipe thread there. As the Ford became a world success, the pipe thread plugs were made available in all countries, so the metric thread head engines weren't built after 1920.