A 1.21 is a good carb and will work. If you can find a smaller one it would perform better at off-idle and lower rpm cruise.
The 1.21 carb has 351cfm, the 1.14=300 cfm, the 1.08=287cfm.
A 200 engine draws about 200 cfm of air @ 4000 rpm. Using the smallest carb for the application has the advantages of better metering, economy, and low-end torque and response because the air is moving quickly over the venturi. The "small limit" is full throttle power, as the carb is nearing it's max flow capacity vacuum increases, and max potential power is restricted. If your car is a gentle-drive cruiser the smaller carbs will be more effective.
As has been said, the Rochester 2 barrel carbs have the advantage of having a smaller mounting footprint than the Ford carbs.