Sergio wants to know how to put an older twist type head on a newer articulated neck. Since I recently got some of the older bodies at the Thrift Store I was able to experiment. Neither of us tried this before, so if you know a better way, please let him know in his post, "Failure."
Sergio, here is my attempt:
You had only cut a hole around the small protruding section of the older neck and as you noted, it's not going to work. Go ahead and cut out the entire area. There is a nice little ridge around the bottom of the neck - slide your exacto immediately before this ridge and cut all around. If you cut it too thin, I'll tell you how to fix this later.
Next, place your head on a thick piece of cardboard and using the neck as a guide, pencil around it. Draw a little circle in the middle and a line going from it to the edge. Cut this out, but leave a little bit of extra cardboard past your circle, where you cut the slit. Twist your cardboard disc open enough so that it slides under the articulated neck knob. Once you get it fitted properly, put some tape around the slit.
Put your old head onto the new knob at a backward angle like so, making sure your slit with the extra portion of cardboard is in the front. Then, holding the back of his neck, push his head down and forward until you cant see the cardboard any more. Squiggle it down just a littttttle bit more (too far and you might split the neck). Now, gently pull it back up so that it's in a "natural" location on his neck.
If you made your disc wide enough, it will get caught inside his chin and jaws and should look and move just like any other articulated head. If you cut your neck hole too thin and his neck tears open in places, then take it off and put super glue INSIDE the neck where those fissures occurred.
An alternative to using a Ken body is using a Liv Jake body. They have a really wide neck knob. So much so, that after you've cut out the opening in the older head, you should be able to just plop it on without making a cardboard disc. I'd make one anyway, just to be on the safe side
Good luck, Sergio!