I have to disagree with you guys, ported vacuum is wrong in theory and in my experience it is wrong in performance.

Manifold vacuum is vacuum under the throttle blades. As idle this vacuum is very high. At full throttle it is zero, or near zero.

Ported vacuum is the vacuum above the throttle blades. At idle it is zero and the more air that flows through the venturis the higher it gets. You can curve a dizzy to work with it, but why? Ported vacuum is part of emissions control because more advanced timing causes a dirtier exhaust charge. When you hook up to ported vacuum it causes your engine to run on whatever initial you set it at, lets say 8. The engine runs crappy on 8 degrees of initial. It will idle rough and have very poor response. Lets say theres 24 degrees of mechanical advance in the dizzy. When youre driving normally its just adding timing based on how much air is flowing through the carb, basically the rpm. At WOT at the top of the gear its adding timing, lets say 10 degrees. So you have 8 initial (not enough), plus 24 mechanical, plus the 10 from the ported vacuum. You now have 42 degrees of timing, which is too much and if it pings you have to retard it so the engine runs even crappier at idle, although it will smell cleaner. Also at high RPM the engine doesnt like timing as much, the extra timing will hold the engine back and make it not want to wind up.

Manifold vacuum basically works opposite. It will add the most timing at idle. Your 8 initial is increased 10 by the vacuum advance and you now have 18 initial. At WOT you will have your 8 initial plus 24 mechanical so you have 32. Crusing down the highway at light throttle the vacuum advance will add a few degrees in and give you better gas mileage and part throttle response as far as maintaining speed goes.

If you run ported vacuum you may as well just recurve the distributor to add in whatever advance the vacuum advance is adding in my eyes.

This is my veiw on the vacuum advance. Maybe I'm wrong, maybe there is no right answer. But I know it works better for me this way.

What engine is this and what cam? And do you have power brakes?