Using ND filter and flash
I would set camera exposure to slightly underexpose (-0.5 to -1) = exposure on the face of the backlit person and then set the flash to standard +-0 exposure. To make the flash expose correctly and exactly on the face with ETTL: use FEL = spot meter with pre-flash on the face, do this carefully! If your camera fails to take this reading correctly then use manually adjusted power settings. (Standard ETTL metering with evaluative measurement will not to my experience give very accurate measurements in strong direct backlit scenes, unfortunately).
A direct backlit scene will typically be totally blown out with this strategy and the way to handle this is to change angle a bit, have your subject backlit from one side as it is always tricky to aim camera in direction of the sun anyway. With Speedlite 580 you have HSS so you can use wide apertures and ignore the sync speed. Power should be no problem unless you are too far away (the dark side of a backlit face is often quite dark ….. so there is no need compete with the power of the sun unless you want to match exposure on both the background and face in a straight backlit scene …… again experiment with angle to the sun). I would never use ND filters, they as they cripple your AF-system. And yes, experiment…. Try it out before they come.