Bolt Moment Calculation

There isn't really enough information here. The real problem is to the left of the bolt, where the thinner member emerges from beneath the thicker member. We need dimensions of everything.

In the assumption that you have ONLY 1 bolt its pre load has to be > F*b/a where F is the maximal possible load. As you see in the way you fix there is an amplification factor which can lead to big bolts. The segment "a" must be as big as possible. If you have several bolts the problem is more complex since it has to be considered how the bolts will stretch and how the fixed part will bend. If you are afraid of a head bending which is in some situation possible and which is also very dangerous for the reliability you could reduce the risk with a long bolt in a piece of pipe. The long shaft will reduce the bending stiffness and thus the head breaking risk. Do not forget bolts are to be loaded only in axial direction any bending must be avoided.

GA to nickname.

The "pivot point" (z) should be to the right where the two moment arms (distances "a" and "b" coincide (where poster nickname depicts them), which is the most conservative approach to obtain the maximum prying force (tensile force).

Also, forgot to elaborate further on what fellow member tornado has posted. He's absolutely correct in stating that the thinner member should be checked.

Both shear stresses and stresses due to applied bending moments in the thinner member should be checked right at the bolt location centerline.

i'm not sure of your intentions? why not just re-design the connection?