Stu, welcome. As far as the positioning of the QSs, as you may know a common suggestion is about 6' up on the side walls and slightly behind the seating position. With your room setup it will be fine to mount them on the back wall and widely separated; not almost to the sidewalls, since the room is so wide, but separated at least 15' and possibly 20', since they are side surounds, not back surrounds.

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Enjoy the music, not the equipment.