You can’t size a carb based on flow numbers alone. The problem with flow numbers for both heads and carbs is that they are tested for steady state flow. (Where the air simply flows continuously in one direction). In a running engine, airflow is starting and stopping very quickly and it is vital that you keep the velocities as high as possible.
If the flow velocity drops, you loose the ram effect from the rapidly moving air column and can loose 10% or more in power just from that. That is why if you have two heads with the same flow numbers, the one with the smaller ports will usually make more power.
With a carb you have the issue that it has to meter fuel accurately and in such a way that the fuel is well mixed with the air. This again requires a pressure drop through the venturi. Too large a venturi will loose horsepower because of a poor mixture. (There is also the issue that you want good part-throttle throttle response and a decent idle).
If you go back to your last thread, I gave you the formula for figuring venturi size. It suggests that 44 mm to 45 mm would be ideal. Since Weber only offers 44 mm and 46 mm venturis (and the 46s don’t work well at lower speeds) you would be best to start with the 44s.
Finally, realize that all of this only serves to get you reasonably close. If you want to maximize the performance, you have to experiment with the engine. One of the great things about Webers is that you can change their size by changing venturis. Larger Venturis will (up to a point) give you more peak power, but will hurt low speed throttle response, etc. You have to test and find what works best for your particular engine in your car and for the way you drive it.
