Have you had a look at the old Thule gutter mount feet? They were a semi similar design, but instead of a bolt on the top there was a sleeve that the bar slid through. I'm bereft of knowledge on steel, but is there a way that a similar box could be included so that the cross bar is located within that, therefore reducing shear load on a bolt that holds the bar in place.
Also is it worth putting a small supporting brace underneath from the outer most point to the upright so that the load is spread onto the bracket better?