I think you should take your M8 studs to an engineering company and get them to drill and tap the M6 thread on a lathe to keep then concentric and square, this will add to cost but trying to drill them in situ will not be easy, the core diameter of the stud is only about 6.8mm in the first place.
Locitite 270 should hold them in fine, degrease everything with a solvent cleaner or petrol first.
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