Vacuum advance of a distributor connected to a ported vacuum source is a retarded Ford thing. Don't do it. You do not want added vacuum advance when an engine is under load. its getting its mechanical advance depending on rpm and the advance springs. Vacuum advance is more of an emissions thing. On GM vehicles its used to advance the timing when the engine isn't under any load or in deceleration that's why GM used a manifold vacuum source. I personally don't even use vacuum advance on any of these cars. just set the initial ignition timing to 15-18 degrees and check the total isn't more than 36 degrees and your good to go.
