IMHO the extended tip (NGK "P") is preferable, and will run slightly cooler as the tip is in the intake wash. However, it must clear the piston dome not only with safety, but with enough room for the flame kernel to expand away from the gap without obstruction.
Try the Bosch quad platinum 4479. They are 14mm, long reach with a very broad heat range. I've been running them in my '40 knuck on the street for about 5 years and never fouled one. Thats saying something for a stroked knuck.