So in the big picture there are dual plane manifolds and single plane manifolds. The manifold that you have tries to be both - there is a reason that there is no current manifold on the market that uses that design. A dual plane manifold has 2 plenums under the carb - one feeds every other cylinder in your firing order, and the other side feeds the other half. A single plane manifold feed all the cylinders from one big plenum under the carb. Your manifold feeds the driver side cylinders from one plenum and the passenger side cylinders from the other side. It would take quite a bit of time and writing to describe the advantages of each (reversion pulses, separation of draws on air fuel mixture, la la la la), but generally single plane manifolds show their strength at higher rpms, and dual planes have real advantages at low and mid range rpms. That means that for a street car, hot rodding around, wanting to punch it from low rpms and feel seat of the pants acceleration the dual plane is best, at the expense of quarter mile time slips if you are willing to really wind that motor up. Your Offy manifold is a cool piece to hang on the garage wall, but I think that the factory iron 4bbl manifold (which was a dual plane) would do as well or better. You can identify a dual plane because the manifold under the carb is split in two, and each pair of runners at the heads has one runner that goes high and one that goes low - all the low runners end up in the deeper of the two plenums under the carb, and all the high runners go to the shallower plenum.
