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Jon.
If the major diameter of what's already there is well over 5mm, an M6 thread will be weak as the proper drill size is 5.0mm. I tap oversized holes out to M6 all the time for keeping my wheels in the frame, but they're not significantly loaded along the thread axis and the holes are only about 0.2mm oversize. You'll have to make your own judgement about how much oversize the hole is and how the load will interact with the ensuing reduced flank contact.
You could use a fluteless (thread forging) tap, but they are expensive to buy and hard to use. The correct pilot hole for a fluteless M6×1.0 tap is 5.5mm