I used to mess around a lot with carb distribution at GM during the carburetor days (when was that?!?).  Anyway, with the intake manifold upside-down in the O-320 some of the car tricks might not work - like dams on the floor of the manifold.  What would probably work is what we called a "dip tube", or in this case a "riser tube".  It would be a tube that fits snug in the manifold with a flange at the bottom to retain it.  It would rise to some distance above the bottom of the individual runners.  From there your imagination can have free reign - add notches, slots, bevels etc until the fuel distribution is evened out as much as possible.  The sharp edge and the slight blockage of the runners will have negligible effect on power since the air doesn't follow the sharp corner anyway.  Another sure-fire method is to simply space the carburetor further away, increasing the time allowed for fuel to mix and vaporize.  As mentioned before, adding heat helps.  Another thing often overlooked is to tweak the carburetor inlet.  Uniform flow around the periphery is the obvious thing to go for, but sometimes a "correct" unbalance will help, created by a tab or obstruction in the right place.

Gary Casey