beware of vignetting: a standard 77mm filter will vignette on the wide end on that lens, but most "slim" filters don't allow you to place the lens cap unless you take the filter out first; I don't know how it will be with a matte box, but with screw-ins I don't think you can avoid this tradeoff
Well obviously you want the cloud to show up . I wrote stop down as in stop down the aperture. The point being you don't need ND filters to darken your exposure and on a wide you very rarely want shallow depth of field so shooting wide open with ND's is counter productive
Well ND filters are certainly a valid concern if you want to shoot with the lens wide open.
But more times than not I find on wide shot exteriors, an ND grad filter is the more practical solution.
I was thinking that the shot composition had a horizon broken by the figure of an actor – in which case a grad ND would give the actor a grad exposure. But yes David W Jones is right, provided your horizon is not broken by an actor then Grad ND's are good. +1
An ND or ND grad filter should not have any effect on your ability to color correct a shot.
I think maybe you should do a little light reading about the use of neutral density and neutral density graduated filters in photography.