In most cases, a small block chevy does not need a 750cfm unless you plan on using it as a drag car and you're making near 450+hp. The point of going with a larger carb is to give the motor more fuel at higher rpms. In your case, you won't be spending a lot of time in the higher rpms and 600 cfm carb will get you better torque response.
With the setup I suggested to you, matched up with your choice of heads, I would estimate that you would get between 383-400hp range, which is good cause a good engine build would get you 1hp for every cubic inch of motor.