GRIGG: ...would you do anything different if you were making another split nut?
I scrounged a piece of cast iron from an old diesel piston, you could purchase some cast iron bar stock to start with. Cast iron because the nut needs to be broken in half later, cast iron breaks instead of bending
http://www.dura-bar.com/products/gray-iron/index.cfm
The threads are 3"-12, I machined the pitch diameter to the small side of the allowed tolerance to be sure it'd fit on the first go.
The rest of the dimensions you can measure what you have and make it to fit.
I didn't do this, but to do it again I'd first chill the nut in liquid nitrogen. Then quickly split it having previously made two passing wedges to fit in the ID of the nut and when slipped past each other would expand to break the nut. Or, might could clamp half the frozen nut in a vice with smooth flat jaws and whack the other half with a dead blow hammer, might snap off clean, I didn't try this.
Doing it at room temperature in the press caused some deformation at the break and required a little filing to get it to fit and screw in again, not ideal.
I used a needle thrust bearing part number FNTKF-4872, not original but a nice upgrade. Needs a washer (just thinner than the bearing) made to adapt the the ID of the bearing to the screw diameter which keeps new bearing centered.
Good luck,
Grigg