Appears to be 1-5/8" (1/16" oversize) on the throttle side, bigger at the air-horn. I've heard that the 4 bolt has been bored to take 1-11/16" throttle disc but never seen it. Going to 1-3/4" may expose the idle/transition passages.
That venturi was also available as a casting that extended out of the air-horn to form an extension.
Not only will this require richer main circuit, but probably a larger transition hole, and an intermediate hole in the main nozzle may help.