A friend had to have this done once,& while we drilled the small hole, we sure as didnot grind across the head surface. Using the original head as a guide, we drilled a hole from the side of the head that intersected both the small & the large holes, then made a small brass plug for the head, which we peened in. This duplicates the way the original head is, because there should be a brass plug in the side. This modification should be done in a drill press & shouldn't go further than the first large hole you get to. You will probably have the rocker gear seized as well, you can save this by using a fine grade of emery cloth to polish the shaft after knocking off any flashing.