If the spindle has a stubby male taper, it's likely a Jacob's taper, and you can Google a chart giving dimensions to see which one. The chuck must have the mating taper. If the chuck you have has a threaded ID, it's not the correct chuck for that drill press.
It's possible that the spindle has a Morse taper, and there is a Jacobs taper spindle adapter in it and you could simply swap out the adapter to a threaded adapter. With the quill extended, is there a slot thru it to knock out an adapter? Would be about 3/8" wide and about an inch long, with rounded ends. It goes all the way thru the quill about where the end of the spindle adapter would be.