Well, those are 7/16 bolts through there.  I tried a M12 tap and it is too loose.  So more options would be to tap it for 1/2-20 or find a really deep nut and weld it on to the back side of the frame and tie it in with all the frame bracing.  These foot peg areas and cushion lever area are going to get really beefed up.   Still debating on what will be the best.