Not sure what your plan is with regards to tapping the holes – if you want threaded holes then you need to drill as below, then use the appropriate tap
M10 – 8.5 mm
M12 – 10.25 mm
The above is for standard pitch
Have you thought about the thickness of the spacer & offset of the alloys? In my little world, thread engagement should be a minimum of 1 x diameter (I think ASME allow 0.75 but I’m not so brave), although for this stuff I would probably go at least 2 x diameter so about 24 mm.
If you are not using threaded holes then holes for M10 bolts should be 11 mm to clear and M12 should be 14 mm.
http://www.thomsonrail.com/Technical Resources/A short guide to metric nuts and bolts.pdf