there are even different carbs. you either have a Rochester B or a Carter YF (or a very slight chance on some oddball thing).
which air cleaner did you get? one specifically listed for this engine, from one of the classic chevy parts vendors?
I don't have my books here to look up sizes, but start checking which carb you have. the factory put a B on it


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1959 Chevrolet Apache 38 Panel Truck in the Gallery