I run dual 600cfm Edelbrocks on my small block. I disconnected the secondaries, not because of overcarburation, but rather because the flapper valves would open too soon and cause a flat spot during accelleration. Anyway I wonder the secondaries open at all with a nominal 1200cfm carburation on a small block.
The plaugs are rather white than black, because of the general lean jetting of Edelbrock carbs.
I think you can't go wrong with dual 600cfm vacuum secondary carbs on a 428.
-Simon