Are you building a competition engine, or a street engine?
You always want the full height plenum divider for a street engine, but especially for a Cleveland with 4V cylinder heads. You don't want to change the induction system in any way that would bump the power band to higher rpm.
If I were building a competition engine I wouldn't purchase a Blue Thunder with a cut-down plenum divider, I'd purchase a single plane manifold.
Blocking the exhaust is easy to do, you don't need to buy a manifold with that feature. They sell gaskets that do that! Blocking exhaust heat makes more power ... but makes having annular boosters in the carburetor more of a necessity to realize the full benefit and achieve the best possible drivability with a cold manifold. The second consideration here is which is more important to you, more power or driving in cold weather?
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