Long story short: I came upon two nice five inch Hardinge lathe chucks; a three jaw and a four jaw. Neither, of course will fit on my 9A.
I also have a B&S #10 dividing head. It has a spindle of 2.25x 4.5 tpi. I'm thinking to bore and thread the chucks to fit the dividing head and turn a spigot to fit them and the spindle of my lathe.
To that end, I'm looking for a chunk of cast iron 2.5d x 2 long.