Graham, Lets iron out a thing or two here. The lingo between oceans is going to get things mixed up.
What is the exact name of this single plane "Edelbrock street performer" you have now?
What is the exact name of the dual plane intake manifold you have.
The lack of a quick rev can be a multi issue problem. We'll get to that in a min. Lets just get things squared away first. A 600 cfm carb is not an end all item.
Well get to timing in a second as well.