You should be able to bore and face in one operation.

Face the plate. Drill the hole to 7 or 7.5mm then ream or bore it to 8.0mm - boring will give you a straight hole at right angles to the face. (if you haven't a boring bar just grind and old drill bit)

Trying to face a 75mm plate mounted on a 8mm bar with a grub-screw sounds like a recipe for failure to me.