In post #22..with the original jaw shape..
Just take two of the jaw screw out and replace them with studs and nuts, the studs will have a nut to lock the jaw tight in the chuck and the remaining stud length would become a simple hold down to push on the part step
The outer stud might be the post and the inner stud might be the hold-down to push a space toward the part shoulder.
Need 3 long studs, 3 longer studs, 9 nuts 3 washers (as the 3 tilt bridges) 3 pusher spacers for a 3 jaw chuck.
* Somebody who understands what I am saying might sketch it and post the sketch. just a simple (3) hold down
Buck