As you know, I'm not much of a mechanic so I'm hesitant to offer advice.
BUT.... You can probably fab a drill guide out of aluminum bar stock... use one of the other head bolts to hold it in place.
Drill-press accuracy is probably good enough. But any machine shop (or DIYer) ought to be able to make one for you for not much cash. They could do a steel one easier than you can at home on a drill press.
Use an oversize hole at the clamping bolt so you have some wiggle-room.
Maybe try an undersize drill bit. Maybe even a left-handed bit if you can find one.
Good luck and let us know how it goes!
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