I'll have to dig around for the vacuum gauge in the garage. I saw it somewhere before...
Everything else you mentioned doesn't seem like a likely culprit. It all came off of the 305 that was in there, and it was running and idling great. Cap and rotor are new, wires are new, plugs new, fuel filter is new, fuel pump is new and the carb has ran great on two different motors.
The vacuum advance is connected to a ported source on the carb. When I set the initial timing, it was with the vacuum advance disconnected, line plugged, in gear and at 500 RPM.
Did you want me to check the difference in initial timing at idle, with the vac advance connected, then disconnected?
I'll definitely get back to you with the manifold vacuum readings at idle.