Thread Milling. Works great for large shallow threads. Tooling would be cheaper, HP requirements a lot less, size and depth control outstanding.
You can single point with a boring bar to prove the concept then buy a thread mill when you want to speed things up.
Start at the bottom of the hole and helical interpolate out. Haven't done it with Mastercam, but in Surfcam it's a breeze, can't be much different.