I just made one for the hardy hole on my old Hay Budden anvil. Find a large ball bearing, heat with a torch until a magnet will not stick anymore, ( a little hotter than red hot ) and let it air cool. Weld to the shank with 10018 for DC or 11018 with AC, preheat to about 500 degrees before welding so the weld won't crack, and again, let the welding job air cool. End result tough as nails, pound on it all day long. You can polish after cooling if you like, but not necessary. My first one was 1 1/2" ball, the next will be a 2 1/2", for me, that will be enough for my needs. Looks like in your case you are better off having a machinist just turn the size ball you need with the size shank you want, and go from there. Costly if you have to pay to have it done, easy job for the machinist though. I just step over to the lathe and do it if needed.
Chris