Sean Davidson has put together a NetBSD/pmax distribution of X11R6.3.
The tarballs are in

    ftp.netbsd.org:~ftp/pub/NetBSD/arch/pmax/ported-software/x11r6.3/

along with a README file and checksums.  The X servers in this
snapshot are still the statically-linked X11R5 servers made by Arne
Juul, though we're making some progress on adding the "ws" API used by
the X11R6.3 Xservers on DEC hardware.

These all seem to work fine for me on the 1997-03-16 snapshot.  I've
finally given up using the X Consortium X11R6 clients, compiled under
Ultrix and running in Ultrix compatibility mode, that are all I've
ever used on NetBSD/pmax.

--Jonathan