[DanEC]

I don't understand why, but possibly slotting the bolt holes in the starter mounting bosses may be the only way out. I would be concerned about the starter eventually backing over however. I guess another approach might be to offset drill or carefully file out the bolt holes on the starter for the next size metric or SAE bolt and ream and tap the holes in the block plate to match. How many bolts secure it to the block plate - 2 or 3? If only two you might get away with only off-set drilling one hole only to swing the starter in slightly.

Have you talked with MSD? I wonder if they possibly have some different gear diameter options.