just shim the rear bushing and use a 14mm thru-bolt. No the 'nut' side (forward face of ear) of the casting is not flat. Nor clearanced for a M14x1.5 standard hex nut. I would not change the bolt to M16.just need to pick up a good HSS 16mm bit, cover is aluminum it should drill out easily. I’ll check the forward face of the ears for flatness and square with the hole, may need to be cleaned up a bit to mate well with a washer and nut.
The correct answer is to shim the bushing or better yet just rip them all out and put in Delrin or WhiteLine's kit.