In 69 the cowl plenum was available through the parts dept. I know of a couple of early built original 69 Z28's that the owners installed the cowl plenum. One of the cars was an excellent fathom green car out of West Chester, Pa.
As William points out it had to be on a four barrel car. I've only seen it on SS, Z28 and part of the BOM on COPO's and Pace Cars.