oops, looks like you saw my non-informative post before i deteled it for being non-informative.
i basically cut off the metal rod of the body's neck and drilled a hole in the head's ball joint, keeping an angle of the drilled hole far from vertical (otherwise it would look very bad, making the head look down, beause that metal rod is not vertical in a normal neck position).
i didn't do it accurately or measured everything precisely, so i intend to redo it better with another head and body, which i keep postponed for those times.
yes everything inside the head stayed intact. that was the goal.