According to that chart I should be ok using a 10.5mm drill on an M10 hole to then insert an M12 X 1.5 bolt.
I was going to reply earlier but had to go to Drs.Ouch. Nasty.
The problem with that sort of setup is that the stronger you make the bolt the more likely it is you're just shifting the weak spot elsewhere. I would give it a go again with the same size bolt as per original but try to minimise relative movement of the two brackets. Ideally you would pin them together but with so little material around the bolt it is going to be an exercise in lateral thinking. How about a split dowel collar around the bolt that locates the two brackets against each other so the bolt only has to deal with clamping? Involves machining, I know, but that is what I would try. Enlarge a bit the hole around the bolt, say to a depth of 5mm, enlarge the hole in the other bracket likewise and use a tight fit collar say about 1mm wall thickness. These collar dowels are used in yesteryear Renaults of all sorts, shouldn't be too hard to find one. There's quite a few for instance in a R12 or R17 and the likes (engine to g'box, starter to clutch cover, clutch cover to g'box, another one locates head to block, etc).