Good start Mike, a couple of things: How did you hold the horn to drill the spout and do the other work? Once you've got it drilled and are ready to tap it for the threader put a small hose clamp on the tip and tighten it down good, it will prevent the tip form splitting. You may need to build it up with some tape so it doesn't try to slide off. What did you use for a lube with the die? Screw a nut on the threads to protect them as you work the rest of the horn, it will also work as a depth gauge to slim down the horn in the neck. You must have quite a swing on that lathe to turn a horn, mine doesn't so I do all that by hand. That's where the 3/4" hole saw comes in handy. It fits a 5/8-11 die perfectly and is easily worked down to 1/2 with a rasp if you want to use a smaller die.
Look forward to seeing it progress.
Tim