If you are going to go to the trouble of removing the panhard bar and substitute a Watts linkage in its place it would only make sense to allow it to be as adjustable as possible in order to optimize the geometry. And as a lateral locating device I can fully understand Kelly's mention regarding surface area. If the material properties/size of the steel used, the location and torque figure of the fasteners, and the estimated load calcs (plus whatever safety factor) are all correct, the axle bracket and mount should be fine.