I think I got it---
When the vacuum advance is hooked up, it shouldn't change at idle. Once the throttle is engaged, and your vacuum rises, that when it is supposed to advance timing.
As for the idle issue, the engine is supposed to run at a slightly higher RPM while in park. When it is in gear, it is supposed to drop a little but. Now Im not sure what you pontiac motor is supposed to idle at, but I have a stroker 350 that idles at 750 while in park, then drops to about 5-550. It dosent sound like anything is wrong.