How'd you know I had the info info in front of me?
The bolt is actually 12mm. You could use a 7/16 I guess, but it will be about half a 1/16 size too small, if that makes sense. The mounts for the bolt are about 3-1/16 wide so make sure to get one at least 3-1/2 wide or more for the nut. I also believe that everything is grade 8.
While you are under there, could you check to see how hard it would be to drill the forward UCA frame holes. I'd like to take them out to 1/2" but I kind of doubt that there is enough room to get a drill in there with a 1/2 bit.