Header bolts stripped

You might be better off trying to go to the next metric size since metric bolts are sizes that are in between SAE bolts. This way there's very little to drill out of the hole. Sometimes the stripped SAE bolt hole is the exact size (tap drill size) for the next size metric bolt. When you drill out the hole you have to make sure and keep the drill bit 90 degrees (vertically and horizontally) to the head surface. Even a slight angle will make the hole not square to the head and will also make the threads crooked. They make small 90 degree drills, you can't do it by hand!!!. You can go to the next size SAE bolt but then you're going to have to cut out more metal and then maybe drill out your header holes. When you tap the hole you also have to keep the tap at 90 degrees to the head surface. Also, once you drill out the hole (Tap drill size) put a small chamfer at the entrance of the hole so that the tap starts with less force. The chamfer will also help to center the tap in the hole.
If I lived close by I would do it for you.

Treblig