A panhard bar goes from the frame on one end, to a bracket at the far end of the rear axle. A bar only 1/2 as long as the distance across the frame would be much too short, the tight arc it would trace will cause side to side motion of the rear end. It should be as long as possible and horizontal at rest. That requires a long mount on the frame end which wants to twist the frame. A diagonal brace should be added from the lower end of the frame mount to the frame on the other side. A second brace just above the first from frame rail to frame rail will complete a triangular bracing and prevent frame damage.