Place chicken in a heavy-duty, zip-top plastic bag; pour 1/4 cup dressing over chicken. Seal bag, and shake until chicken is well coated. Marinate in refrigerator 1 hour, turning bag once.
Coat French bread cubes with cooking spray; place in a single layer on a baking sheet. Bake at 350° for 10 minutes or until lightly browned.
Remove chicken from marinade, discarding marinade. Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place chicken on rack; grill, covered, 5 minutes on each side or until done. Let cool slightly. Cut chicken crosswise into 1/4-inch-thick slices.
Combine chicken, bread cubes, lettuce, and next 3 ingredients in a large serving bowl. Pour remaining 1/4 cup dressing over lettuce mixture, and toss well. Sprinkle with freshly ground pepper, if desired. Serve immediately.