Unfortunately using a z-bar type link like that for the steering will not help with the bump steer. It will help to keep the Heims from maxing out from misalignment, in turn relieving some of the stress on the joints, steering box and arms when the axle droops down. However the effective angle of that link even with the z in it, is the same as a straight line drawn through the two mounting points. Being such a steep angle, as the axle droops or compresses the drag link will still push and pull on the steering box.
A longer drop pitman arm will help with all of the above because it will reduce the angle of the drag link by bringing one of the link mounting points closer to horizontal. The closer to horizontal the drag link is, the less bump steer you will have. The trade off here is that it puts even more stress on the steering box, pitman arm and frame mounting.
You are running some pretty steep angles. My suggestion would be to build a stout drop pitman arm, a stout steering arm and reinforce it to the knuckle to get the drag link as close to horizontal as possible (doesn't have to be perfect but definitely better than what is there currently). Also run hydro assist ram on the axle. Those tires and steering angles are putting A LOT of extra stress on all the steering components, joints, knuckles, steering box, steering box bearings and frame mounting. The hydro assist would greatly reduce that extra stress.
Just my .02