@jmorris Glad you got it working!
Tap guides are a reasonable form of vertical reference; mill not required. That's why I mentioned what I do for tapping v-slot. They keep the taps straight enough that I've (touch wood) had 100% success power tapping with a drill and a spiral flute tap.
Also, not trying to talk you out of form taps. I want to go in the opposite direction and start using form taps where appropriate.☺