Thanks @visionaut & @Mike G. I think I answered my own question. Since there are only two studs for the master cylinder to mount onto the booster, the port can only be at the top, or at the bottom. When I tried to mount the booster with the vacuum port up, it was obvious that the elbow that feeds the master cylinder from the reservoir was going to interfere with the port. In other words, unless you are willing to do some non-obvious mods, the vacuum port needs to be at the bottom.
m