The old J8 Champion plug is what mostly came from the factory, on the 14mm head. I have moved a lot of those. Now I generally supply Autolite #295 for those that want an OEM plug, with a better performance.
Right now the #437 Autolite has shown a slight edge in performance. The plug extends a sixteenth further into the engine. It is a power tip plug and fires off the trash that passes for fuel, these days. The #216 Autolite is a cooler plug that will give a slight horsepower edge, on a dyno, while giving the extended reach of the #437.
If you have 18mm holes to fill, I always reccomend the Autolite #386 plug.