Ummm, didn't some of the high performance 383's get the 750? I thought so. I know the regular 383's had the smaller carb.
The 67-440 in my freinds Imperial had the small AFB.
'68-'70 383 hp had the same cfm carter avs carbs as the 340s had. I can't speak for the other years/models. Only the 440 got the bigger carb during those three years.