I am using manifold vac for both booster and distributor. Here is s closeup of the rear carb which has the ported vacuum take off, it's the one that is capped off with the black plug (pointing away at 5 o'clock) and is separated from the manifold port (braided hose) by the throttle plate. Each carb has a manifold vac port on each barrel which I use for balancing.
#5 barrel vacuum port is located just to the right and below the idle mixture tower and is capped:
So the distributor can be used with either manifold or ported vacuum, I use manifold because I want the extra advance at idle and I don't want extra advance when accelerating. From my readings the auto industry switched to ported vacuum solely for emissions reasons and that manifold vac was always used prior to that. At cruise and WOT, manifold and ported vacuum are equal.
If you want manifold vacuum with DCOE you could just tap one of the intake runners, for example #6 so it is out of the way and get the anti pulse valve to help smooth out the signal. Also depends on which runner the booster tap is on, I've seen different tapped runners depending on intake manifold manufacturer and how it's installed. Personally I like booster vac off #1 port for ease of connection to the booster and #6 for the vac advance, this keeps the whole rig a little cleaner visually. Or like you say you could just tee into the booster source.
Steve, from these 2 pictures, the booster is using manifold vac and it seems like you are using vac from just downstream of the throttle plate for your distributor. These are a little different in their characteristics (I think).
THanks for the pics.