Thanks for the comments.  El, I set out to just swap arms, so I was trying to get away with changing as little as possible.  Also, I thought the z400 bolt pattern was different, so if I changed hubs, I wouldn't have any front wheels that fit.  Dez, the upper joint does look bad, but there's at least 1" of thread in the tube.  The problem is not in the spindles, it's that z400 lower arms are that much longer than the uppers because the Z has a narrow lower frame.