Are you saying it’s a 1x6 because the drywall is slightly feathered out around the opening? If so then check again as that might just be the corner bead. A 1x6 trim around an opening would typically not be flush to the drywall - it would tend to crack even with mud and tape.

Is the wall 6 1/2” thick from drywall to drywall? If so then that’s at 2x6 framed wall which would only be used in an interior for either plumbing or structural reasons. If it’s structural then the header above the opening will be solid lumber and you’re golden. Just screw the brackets into the header - that could also explain why the stud finder is finding offset studs since it would be picking up the nails not the studs.