seems like the attachments points are not at the same angle between the Z and the FX. if the mounting points on the Z are closer to being parallel with the rear axle than they are on the FX. This would put these welds under tension just by installing them. The fact that no one with the FX specific braces has seen these issues makes me think that it is related to the fact that these braces were fitted from a Z, and not made for the FX. Where the bars bolted in place before the extension pieces were welded in?