This is such a complex subject I could write a book on it. There are a couple of threads that deal with it in great depth, no doubt difficult to find with a search. Vacuum advance is not generally recommended for a performance engine at all. The justified fear being to much total advance under certain conditions, no matter what port you use.
There are basically only TWO types of vacuum on an engine. ABOVE the throttle plates (ported vacuum) and BELOW the throttle plates (intake manifold, direct, unported full vacuum).
Which one to use? Well now, that IS the question, isn't it? There is no right answer. It all depends on what you are trying to accomplish, what you believe (right or wrong) about how the vacuum signal works. Which brings us back to the 'book'...
My personal belief is that there is no 'steadily increasing' vacuum signal ABOVE the throttle plates as the rpm increases. That is, no appreciable increase in vacuum signal ONCE the throttle plates begin to open. There is vacuum, or there is not. Therefore, the second you 'step on the gas' it makes little or no difference WHICH port you use as BOTH ports WILL provide a strong (or weak, as the case may be) vacuum signal.
Which port to use, therefore, becomes a matter of how you want to tune your engine for the best 'idle' condition. Using vacuum below the throttle plates WILL require an adjustment of the idle speed screws (and possibly the air/fuel ratio screws as well). Using PORTED vacuum (above the throttle plates) will not change the idle speed or require mixture screw compensation.
I prefer BELOW the throttle plates myself, the vacuum signal is a 'hair quicker' to respond and slightly more vacuum is present overall. But the REAL reason is so my idle speed can be set SO LOW the engine WILL shut off immediatly when I turn the key off, rather than be subject to a possible 'run on' or 'deiseling' condition. My cam profile requires a rather high idle rpm, 1000 rpm or so. That is 'just fast enough' to cause engine 'run on' in my case. Maybe it's not a problem with your motor...
Tricky stuff this vacuum advance. Ask the BIG dog engine builders and I'll bet every one of them will say 'dont' go there' at all. To set it up correctly it's best left to the pro's. So, some more pictures and data, 'cause, WE are the 'pro's'!
