If you're keeping the stock intake a 650 Holley will be about all the intake can use. Back in the day I'd go to the drag races and look at the carbs on the cars and they were much smaller than I would have guessed. These weren't bracket racers, they would have run a bigger carb if they could. But obviously the slightly smaller carb gave better ETs.

If and when you get your "new engine" figured out it will be time to go bigger on the carb. But you have to realize that 383s came stock with carbs that weren't much bigger than 500 cfm. It used to be that putting on a 440 carb would help out quite a bit, and those 440 carbs were in the 600-650 cfm range.

I can see some big corks in your engine's performance and the carb ain't one.

R.