Hi Yamdude,
to the forum
the brakes for your xs400, if it is an SOHC mode,l your brake caliper is shared with xs750, 850 and 1100 models (all from the right side of course). If you have lugs on your other lower fork you can have dual disks and use all the latters left calipers plus the Sr500 which have their single cliper mount on the left.
if it is the dohc model some xj stuff will fit it.
there is a sticky for xs links, top of classics
drewps