1.Have you set the fuel level corekt in the carbs ?
2. I would try to open the outer carbs whit engine on idel on the center carb and see if the engine responce like it should on those carbs to verify that they give the engine fuel.

I have not done any work on the sb 6-pack but on the bb they like larger main jets for preformance like 66-68.

It shore sounds like the outer carbs dont deliver fuel to the engine so are they assembeld corekt whit the corekt gaskets for the jet plates so the fuel holes aint plugd !?

Jerry Binder wrote a realy good articel that explain alot on the 6-pack carbs that i can email you if you like !?