I was looking through Sports Car Graphic, 11/65, and it lists the comp engine as having aluminum Hi-Riser heads / intake, and a 780 cfm carb, not an 850 like was stated before. The comp car in the article appears to have a carb with mechanical secondaries, as there is no vacuum dashpot for the secondaries.
If I remember correctly, I had thought that I heard that Holley designed the 850 carb, with bigger throttle bores, for the '67 L88 vette. Like you said, maybe the 850 carb was out in 1964/65.
Anyways, here's the graph from weber I was eluding to earlier..
427 has about 875 cc per cylinder.
