"I'd assume then that part of a proper NPT thread callout on a mfg print is to specify the NPT thread OD in the plane the thread is cut into?"
Hydtools, btrublood, and HDS are correct. The head of an NPT hex plug should never meet up with the mating surface. If it does it was machined...
I am not sure this will help, but when I used to do this work on pressure vessel flanges for a local company. I near netted the flanges on a flame table from A572 plate and machined them on a CNC machine with a third axis (turret had live head capabilities which we used to do the hole drilling...
