Should I ground the metal box for 14-50 outlet?
You want one of these



attached with one of these



right here



Then you can lay the bare ground in the lay in lug, no wire nut in the box, no pigtail ... at least IMO that's the easiest way to ground this.  

The green screw is typically used for grounding but you can really use any 10-32 machine screw.  

Be sure to choose a lay-in lug that accepts #6 wire.