Yes I'm aware. The vacuum leak was below the carb, so the carb had to come off.
Before the carburetor goes back on the intake manifold for anything, it is going to be set the way the factory recommends it or until someone can tell me this situation is normal. Nothing from Holley or Demon shows that it is ok to have half the idle transition slot exposed.
As for timing, when the carb goes back on, I'm going to start with an initial 18 like you recommended, but I need to tune the distributor to only allow 16 degrees mechanical advance. I will probably just buy the Pertronix distributor because it has nice easy tabs that let you limit mechanical advance to to 16 degrees. (their tabs allow 12, 16, or 20).