A cowl plenum air cleaner for a small block will have four "windows" in the area that would lead toward the duct. A big block cowl plenum air cleaner will have five of these "windows", and the metal extension with the rubber seal on it will be a little wider and longer. Also, the big block version has a small notched area on the driver's side that provided clearance for a breather cap on the LH valve cover. A notch at the front of the cowl plenum air cleaner is for clearance for the '68 smog pump. The '67 air cleaner does not have a notch up front because the smog pump and hoses have enough room without it.
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