The only way an open plenum under the carb will be an advantage is if you have a "single plane" manifold where all the runners to the cylinders meet in a common open plenum. Most OEM manifolds are "dual plane" where the two center cylinders on one side of the engine and the outboard cylinders of the other bank are fed from one side of the carburetor. The low end and midrange torque of a dual plane manifold engine is measurably better on a dyno than a single plane. Single plane manifolds are made to maximize top end horsepower for guys who never intend to run their engines below about 3000 RPM or so and don't care about low end torque.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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