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
Another (hack method) would be to take some Elmer's glue to make a paste of SG wheel grits and coat the part where the chuck jaw will hit. Then take light cuts so as to not stress the holding.(yet another good reason to own a surface grinder)
A bearing, a friction bar, an angle plate, a 123 block so set off the saddle too rub or just miss the part can apply pressure/safty to a chancy part ..and then take-stock travel can come only from the combination of cross and compound feeds