The story goes that Smokey was asked by GM to make hemi heads that could be used in tandem with the cross ram to produce more power, to help Roger Penske / Mark Donohue during their Trans-Am stint.
Since the results were disappointing, the project was shelved by GM and the five or so sets that Smokey made supposedly went back to GM engineering. It seems a few sets "made it out".
That's the story I read in a magazine article a few months back.
I saw a silver Z (probably the same car) with this setup at the Camaro Nats in Carlisle, and it looked pretty wicked.