The rating was different from holley due to the water column used.
A 1000 flowed = 950 holley, remove choke and co tour the airhorn, dress the blade screws and shaft for more flow...over 1000 cfm.
Further, on open plenum intakes, use a 1" spacer and remove the carb's secondary baffle plate.....if track and stall convertered. I hear it picks up a tenth or 2