I once tried to hook up a vacuum gauge to the rear intake port of my GMC. The gauge needle vibrated wildly every time #5 or 6 cylinder drew vacuum. From that experience I learned it is better to pull vacuum from as close to the center port as possible so the vacuum effects of all 6 cylinders are felt evenly. I run 3 Weber 32/36 carbs on my GMC and use a small tube manifold to combine the metered vacuum off all 3 carbs into one for my vacuum advance signal. It seems to work fine.
