The ATR (and METCO) rear sway bar is often a tight fit but no shaving should be necessary.
Raise the rear of the car by the frame rails and allow the rear axle to hang on the suspension. The axle should be supported by a hydraulic jack to avoid damaging the brake lines. If the sway bar will not slide into place, disconnect the lower shock mounts to allow the rear to hang a bit further and the sway bar should install with minimal effort.