Here is a really good video where different size carbs are compared on a 383 using a dyno:
https://www.youtube.com/watch?v=tjbUd5mrmWg
What I take away from it is that if you are starting from scratch or replacing a worn out carb then the 750 that Geoff suggests is your best bet. However, there is probably little or no difference between that and the unit you already have below 4500RPM and you may well only see an improvement of around 2% in power by going larger.