I did this job recently. It's pretty horrible how I did it, but it worked. I asked my machinist friend for help, and he sent me a few website addresses to order a back plate from. I asked him if he could do it in his shop for me. He said no.
So, I made a "blacksmith thread". It involves taking a strip of sheet metal the size of the circumference of the spindle nose. Lay out diagonal lines with the thread pitch and cut the threads on the flat with a hacksaw and a triangular file. Keep cutting until you nail the minor diameter, and then go a little further. Roll it up carefully and TIG weld it or braze it in place. If it doesn't quite fit, blue it up and touch it with a rotary tool with a cutting disk. You only need one or two threads to hold it on. Do not damage your spindle threads when you are fitting it! It does not have to hold all that well, since you won't be using it for long. Make sure that your spindle threads are well oiled every time you put it on, and don't leave it on. That will prevent it from galling. Note that you still won't have a chuck available, so find another way to get a transfer punch. You can order one, which is seems to be what you did. Or, you can make it on another lathe, or you can hack one up with a power drill and a grinder, like YouTube. Once you have a back plate fitted, you can turn anything you want in the 4 jaw chuck. The first thing you will turn is.............a new back plate with a real single pointed thread and registers on both side that are faced true.
I showed this temporary plate to my machinist friend. He shook his head and told me "you never will become a decent machinist if you keep thinking like a blacksmith."
