Carb CFM recommendations

You are overthinking it. Happens all the time to the best of us. All those trees are preventing you from seeing the forest. You said "moderate performance and responsive drivability" so, you don't have to worry about dyno numbers or ET slips, it's an occasional use cruiser for fun. You could put a two barrel on it and it would still cruise around just fine.
I'd just use the 650, tune it properly, and drive the car. Have fun.