The thing with calculators is it is science. A mathematical equation. It doesn’t take into account the restrictions of the intake manifold or exhaust, much less the build of the engine. The 649 cfm the calculator came up with is one thing but reality will show a larger carb will make more power and perhaps, drive better. Why?
The carb does two things besides give headaches. (LOL)
It mixes air and fuel of course but it is also a variable restriction based on drivers needs for the conditions driven and needed. Under a W.O.T. Setting, it is still a regulatory device. The less restriction through it the more power the engine makes. This is why you see this happening at the track, back streets and on TV shows like engine masters.
Even though the TV show reached a point of diminishing returns, what it is showing is that the less and less restriction placed on top allows easier breathing of the engine and thus more power.
Most people have a hard time and freak out when others say use a larger carb and fear a drop in mileage. Which is t the carbs fault since that aspect is co trolled by the state of tune, or lack of and the amount of pedal pressure given from the driver.
An engine will need a certain amount of air and fuel to accelerate the vehicle at a rate of... 3 feet per second or 60 feet in two seconds etc.... This requires the carb, no matter the size, to be opened a certain amount. Once at a cruising speed, the throttle required to keep a steady speed on the interstate will vary per the size of the carb (or front two barrels on a 4bbl. carb) of a certain amount of area. Which will roughly be equal to each other at the butterflies opening no matter the carb size.
The small carb has an edge on throttle response because of the smaller barrels which will constantly have a greater velocity giving it a crisper response but less acceleration at a certain restriction point (rpm’s) and less top end.
On a street driver, smaller is better for the crisp throttle response, high velocity to mix the air and fuel very well and not really miss the top end power differences which are normally small.
On a street strip or track car, it’s a world of a difference.
IMO, your going to be fine with your final choice you made.
One of the things that took me back and made me say “Hummmm” many years ago was a friends (IIRC) ‘67 Imperial with a 440 that had a choke less 600 AFB on top. Interesting food for thought.