I understand that I was a little late for the party, but I have some interesting observations that I want to share.
I would think the 650 would be plenty.
If you want to be sure, stick a vacuum gauge on it and run it up at wot.
If the vacuum goes over 1" of vacuum, I would go bigger.
What intake are you running? What is your base and total timing? Have you verified WOT? What pressure are you running in the fuel system?
Your fuel pressure should NEVER drop during a run. If it does you are running out of volume. In efi cars I know that a vehicle can loose as much as 50% of it's fuel volume and still show pressure within 2psi of spec. So if your gauge is dropping you have an issue.
How is your fuel system set up? The way I set them up to prevent the regulator from being a restriction is to run the inlet fuel to a fuel log on the carb and put the regulator after the carb. This allows full fuel pump volume to get to the carb.
There are a lot of things that can make a car go flat. Keep giving us data and we'll help you figure it out.
Other question how to choose good carb.
Some advise hot to calculate your carb requirment:
The formula for calculating how much CFM (cubic feet per minute) an engine requires is:
CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.
When you apply this to any engine, consider that the volumetric efficiency of stock is 80%, a slight upgrade with average bolt-on have a volumetric efficiency of around 85%, and race engines will range from 95% up to 110%.
To calculate the CFM requirement of an engine would then look like this:
351W engine x 4,000 max rpm = 1,404,000
1,404,000 x .85 = 1,193,400
1,193,400 ÷ 3456 = 345 CFM
This means that the best stock CFM for a 351 Windsor like this
https://mechanicfaq.com/сarburetors-for-351-windsor/ would be around 350, anything above 400 will deliver performance improvements, anything under will deliver stock performance.