Ensure smooth drywall surface around stone mantel
Cut and set 2 vertical studs screwed to the horizontal stud by bending tabs to attach to the underside of the horizontal stud. You can do the same for the bottom, but bend the tab out so far to find stable concrete or masonry to use concrete screws to attach. The mud bed the hearth will need will allow you to run that tab under the hearth to find good attachment there.