[HO455]

The vacuum advance line on the distributor needs to be hooked to a ported source. There are several reasons for this. It allows the advance to return to the same location everytime you let off of the throttle allowing the idle speed to be more consistent. As engine load affects vacuum and you don't want the that affecting your idle quality.
When the engine is at cruise speed on the road and you let up on the throttle manifold vacuum spikes high with a non ported source the ignition timing maxes out while the engine is experiencing a lean condition making conditions ripe for preignition detonation. With a ported source the timing is reduced when the throttle closes. This is why virtually every auto manufacturer during the carbureted era used a ported vacuum source.