[K.Wilson]

Whaler,
I run two 600 Holleys on mine. The "too much CFM" problem is not really a problem with the vacuum secondaries as long as the carbs are adjusted correctly as they will only open the secondary system as required by the air flow through the card. If they are not adjusted correctly you can have the engine "bog down" from to much carb.
I would not run a mechanical secondary carb on a 2x4 system unless you could find two 400's or 450's and even at that they may not be able to be adjusted to give you a smooth power curve.
Keith