You'll need a left handed 1/2" X 13 die if you want to thread the end. Otherwise, you'll have to drill a hole for the worm bushing and another hole at the end of the shaft to cotter pin it to the clutch drive shaft.
i know a guy in hazelton pa who has worms for around 90 to 110, he has someone who makes them for him. i was making them for awhile but then stoped you need a left hand tap. and a good set of drill bits