Am I missing something or are you actually looking for a bolt size for the applied shear?
First determine the total shear load on each of the individual bolts. You will have greater shear load on the outer bolts than the inner bolts due to the applied torque. You can use vector math on each bolt to determine the applied shear due to the torque and vertical loading. Then back calculate the bolt cross section based on your desired bolt material with the FOS.
You can determine the pre-load with the following.