Welding a square box, eccentricity and one sided fillet weld

[juafh]

Hi, I am welding this square box of approx. 3m down to the deck of a boat with fillet welds with 6mm throat thickness. The plate is bolted to the box. The forces I have applied, Fy and Fx are expected at every corner.
I am mainly concerned about the weld, and what kind of eccentricity I might expect in this? Also, I am unsure about how to deal with the fact that this is more or less a one sided fillet weld, with the other fillet weld being 3 meters away. Could I consider a more "global" configuration, adding together the forces of two and two corners, and then consider it to be a double fillet weld?
Added a picture to show what I did

 

[kingnero]

Can you provide a FBD of the box?
Do all Fx's point towards the same direction? Where does the My come from?
The eccentricity won't cause any serious problems at first sight, on the condition that there will be close to no gaps inbetween the box and the fixation plates... Then the single sided weld might be seriously (over-)loaded. Without knowing all of the details of course, I'm not a big fan of this configuration. Could be made to work though (tolerances, shims, ...).