Here's what I ended up with:
9 degrees initial static timing
Idle timing with VAC connected = 9 + 16 = 25 degrees.
23 degrees of centrifugal advance
Centrifugal advance starts at about 900 rpm, and is all-in (9 + 23 = 32 degrees) at about 2500 rpm. This is a little earlier than I wanted it to be all-in by, but the car doesn't seem to ping or knock under any conditions. Should I mess around with even more springs to bump this up to 2800-3000 rpm??
The car runs great with the timing set-up as above. However, there is one weird issue. I have the idle set at 700 rpm per the manual, but I like the idle a little lower - around 500 rpm. If I try to go below 700 rpm I get a loud whistle from the carb. I contacted Cliff's High Performance, who rebuilt the carb. He said that with my idle timing at about 25 degrees, I have too much idle vacuum (I measured it at 20") and that's what is causing the noise. He says that to get my idle down, I'm closing the primaries too much and the airflow is whistling past them. He suggested that I run ported vacuum to reduce the advance at idle, thus decreasing vacuum.
So, I hooked the VAC back up to ported vacuum and I get a nice idle at 500 rpm, but I haven't driven the car this way. So, if I keep the VAC connected to ported vacuum, what are the downsides of having the idle timing at only 9 degrees instead of 25 degrees? Should I give it a try?
Under all other driving conditions, the distributor advance system should function the same whether the VAC is connected to ported or full manifold vacuum, correct?