While macaroni is still hot, mix with chicken and Italian dressing. Marinate overnight, if possible, or for at least 2 hours. Blend mayonnaise, lemon juice and mustard. Stir in chopped onion, celery, pineapple, and pepper. (Sliced, pitted, ripe olives may be added at this point, if desired.) Add to macaroni and chicken mixture. Salt to taste. Mix well. Chill at least 2 hours to blend flavors. Serve in lettuce cups.