The 64-bit server will be installed in /usr/X11/bin/amd64/Xorg.


To make the Xserver script use it by default, run (as root):


svccfg -s application/x11/x11-server \
   setprop options/server=/usr/X11/bin/amd64/Xorg


To switch back to 32-bit Xorg, run the above with the /amd64 subdir
removed.