As noted by some of the posts, the bottom of the spark plug should be just about flush with the chamber - it should not be recessed/shrouded. This picture (posted above) is how they should look (regardless of head manufacturer). It is important that your plug reach/depth achieves this - but does NOT have any plug threads out in the chamber (as that can cause a hot-spot and detonation).
Surface Gap Plugs: Be very careful in using these - it takes a lot of energy to "jump the gap". I tried them for shits and giggles on my 32 Cab engine with a new Vertex mag on it. They ended up frying the magneto coil . . . so I went back to good ole' NGK plugs.
Original Aftermarket Heads: Companies like Edelbrock and Navarro (back in the day) all used the same plugs as stock Forks --> Champion H10 length (which I believe is 7/16).
Later on, newer versions of their heads (and other manufacturers) switched over to 3/4" length plugs. I always prefer 3/4" length plugs (if the heads I'm using are available with them) - given the HUGE variety of plug choices that are available in this length.
Here is the picture (posted earlier) of how the plugs should look - regardless of manufacturer and chamber design:
IMG_1027.jpg