"Danger, Will Robinson!"- - - - - -If you drill and tap, you're going to be making a big mistake and probably ruining the head. The 5/8-11 adapter has a flat bottomed area for the sensor bulb to seat against, and the straight thread above it. Unless you have a way to machine an adapter with that shape, there won't be any flange for the bulb to tighten against. An aftermarket mechanical gauge with a sensor bulb costs just a few bucks, especially if you get a cheap one on Ebay, and it will have all the adapter hardware you need. Even if you don't use the gauge, you'll be able to install the one you're working with.
Jerry
