Take em' to the shop John. While they have them, ask them to boil them clean and check them for flat. If the threads need repair they will have the stuff right there.

Or, soak them in penetrating oil and use heat. A lot of heat. Turn the fouler a little red. Quench it in wax. Last resort is to continue to drill until you can take a small chisel and collapse the drilled thin wall inward and pick it out of the threads. You definitely want to avoid ruining the threads in the head. If you do, a thread insert will fix it. The biggest danger is drilling it off center. If that happens just don't drill to big. Cut it down until you can start to carefully chisel it out.