It was more common to open up stock castings in the past, there was a big jump in venturi size from 1.375 to 1.56, 950 HP to a 1000 HP. Now Quick Fuel has a 1.40, 1.45, 1.50, and 1.59 venturi sizes for a 1.75 throttle blade. BLP can machine billet any size up to 1.63. I will usually open a 950 to around 1.42 to give it some flow, clean the step in the casting. I'll occasionally open a 1050 when the owner wants to stay with their carb, and engine dictates a larger carb. New builds I just get the right casting or billet size correctly.