Off topic part needed 66 Fairlane

If there's one there but it's bad, send it off to be rebuilt. I had to do so with the booster for my '72. If you're near the west coast, I can fully recommend Power Brake Exchange out of Los Angeles. The booster was out of my possession for less than a week including shipping time, though to be fair, I'm only about a 3-hour drive from LA. Even if shipping is a few days longer each way for you, it'll be less than two weeks based on what my experience was, and you'll lose a lot more time than that trying to hunt down that unobtainium unicorn. I paid $120 or so plus shipping, though that was two years ago, so it could be a bit more now.

Are you staying with the single-bowl brake master, or upgrading to a dual-bowl cylinder as became standard in 1967? If you're upgrading, I'd think a 1967-1969 Comet/Cyclone or Fairlane booster should work. (It's possible the 1966 uses the same booster, but I'd assume the dual-bowl would use a different one.)