The 600cfm will be plenty for your engine.
The 750cfm will more than likely be too much airflow and will be hard to tune at lower RPMs and probably end up costing you alot of torque. If you were running at least a 2,500 stall, really low gears and a solid lifter long duration camshaft.....then the extra airflow of the 750 might be useful and provide a little room to grow as far as your engine combo goes, but realistically a 600 or 650 would give you better results.
