I'll play the flip side of the record.
These trucks were built with fuel economy in mind. Remember the time frame, WWII had just ended, Korea was in the picture. GM intentionally undercarbed the 216/235, so as not to waste gasoline.
More carb will raise your performance on a stock 235. I can't say by how much, but some. If you do go with duals on a stock 235, you will probably need to rejet some to lean out the mixture.
Another good way to improve your performance is to change the exhaust to a split exhaust manifold, either with headers or by having a stock manifold split and welded. Kansas Kustoms does this.
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