hp is torque x rpm, so 750cfm will make more torque and through wider range than the smaller 600 carb after the smaller carbs initial 'snap' when u dip the throttle has past...... See the smaller carb is great for low speed part throttle, that is..for about a second or 2, after that.. the power is flat/flatter than the next step up=700/750.
This doesnt mean ''the bigger the carb, the better''..well actually to an extent it does in theis and many other cases...but, there is a point where too big, and u get poor low speed vac signal to the boosters and the mixture go's to crap till the rpms are high enough...and funny cause then at that point where it gets some rpms.. it will still make more power than the ''small'' carb.lol
There is a size range per cubic inch and rpm level, for your motor imo it's 600-800cfm. So if u would like it to still make torque at rpms over 5000...u should run at least a 700cfm carb,imo