What have you done with the pre-load on the four link bars? If anything. You said new engine is lighter and more horse power, that can change the rotation of the rear end. The tire with the most load will push the car off coarse. in your case to the right meaning there is more load on the left rear. Keep in mind that all chassis are different and will react differently to changes. Also that with previous adjustments with the lower horspower engine might not be the same with the new engine and more horsepower. Good luck