There are trade offs - to really underexpose ambient you need to do one of the following - without going into budgetary concerns:
High speed sync - using HSS capable triggers with about 5-8 speedlites to illuminate for full body. This will help you push your shutter speed higher to kill some of the ambient light, and bring the speedlites more into becoming the main light. Be aware, however, this is going to cost money - so probably renting a few speedlites might work best for this.
More power - You'll need probably around 800-1,200 watts of power (depending on distance to your subject that your light is and your modifier of choice) to be able to overpower the ambient the way you want.
ND/8 stop gradual ND filters - can screw them on to the front of your lens element to help kill some ambient, but your light STILL has to overcome the ND filter, and a 600 RT probably won't be able to swing it. You still need power in this regard.
Shoot at later time of the day when light isn't so hard/harsh or shoot on a heavy overcast day.