Holley rates 2bbls differently. The 2bbl rating for the Six Pack is 1350, 2x500 outboards, 350 center.

To get an idea of the outboard flow rated like a 4bbl, the throttle bore and venturi is the same as a 750DP. So just the 2 outboards total 750cfm. The center 350 is estimated to have a 4bbl rating of 230-240.
Total Six Pack flow is 980-990.