The Carter 964s is the stock number for the carb that chevrolet indicates for the years 1937 through 1952. It came with initial venturi size of 1 1/32" and secondary venturi at 11/16 id. and the main venturi at 1 1/4" id. . I don't know the specs of the 966s but perhaps you can find it in the Carter Carbutetor information for Chevrolet and GM on line. I have been looking at the YF series 2467 to modify for my 235. I believe the 964 is a size 2 and the 2467 is a size 3 so the issue is the venturi sizes and the vacuum. Too big a venturi and the carb will run lean...not a optimum. Bigger is not always better or more effective, less vacuum may not excite the venturi adequately...especially if you are looking at dual carbs. I know there are some pretty carb wise guys out there so I am pretty interested in some replies about this as I am tinkering around myself.