1989 SEL 560
I leave soon for a trip with my brother. That vehicle will now sit for a month.Liviu165 is right on the timesert but also it not only needs a chamfer but the surface of the thing you are doing needs to be countersunk with a special tool so the gasket will seal. For what you are doing I would stay with a helicoil. I am sure you have enough meat on the head to fix things. Keep us in the loop. Actually I think it was timesert that had the oversize insert, not helicoil.
I have enough time to investigate something I read about on my parts vehicle head. A saturated solution of Alum (ammonium aluminum sulfate) is supposed to dissolve ferrous bolts without damaging the aluminum head. I will try this on the same head bolt that also sheared on my parts vehicle. The head is sitting on my workbench so I only need to go to the grocery store to see if McCormick Alum has the correct chemical composition. If not then I will buy some off of eBay.
I still have M10 helicoil and the head from my parts vehicle as options for me.
I'll update the board sometime in June.