The competition cars came with a single carb for ONE specific reason, it's impossible to mount two center pivot float design carbs back to back! At Le Mans when Shelby was looking for more horse power he went with dual carbs. But for a short road course the center pivot floats are a must to avoid fuel slosh under hard cornering G forces. The long straight at Le Mans was the primary concern and for top speed (180 plus) he needed more power. The typical competition car was for short track, extreme low gear ratio and top speed generally not in excess of 130, 150 max (at a million rpm).
Dual fours WILL make more horse power AND can potentially deliver BETTER fuel economy due to a better fuel distribution in the intake. Two small primaries feeding front and back are more effecient than a single larger primary feeding center.
Having said all that... me? I'd go with a single CENTER PIVOT float 750 Holley for better all around street/short course road handling. If I couldn't run center pivot, for what ever reason? Two vacuum secondary carbs abut 450-500 cfm per unit, adjust the float level on the 'low side' and get the power valves to match the cam profile, probably around 8-9" vacuum or so should do it. I think mine are around 6-7" because my cam has a good bit of 'lope' to it. Dual fours here. Max horse power, like someone said, no worries, it will have more than you can handle not matter which way you go.
Single carb would make more horse power than duals IF:
1. Your over carb'd with two, to rich kills the power.
2. The duals are harder to set up and make work together, single is MUCH easier to diagnose/setup.
3. For the above reasons if I was your engine builder I'd highly recommend a single four, just to make life easier on myself.
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