Open a large zip lock bag, pierce chicken in several places to allow marinade to penetrate, then place chicken inside.
Add 12 ounces of dressing folding bag in half before zipping to force air out.
Place bag in dish to avoid leakage, and place in fridge for at least 4 hours. Drain chicken, discard dressing and bag.
Heat the grill. Make sure grill is clean and oiled, or use Reynolds BBQ strips.
Grill chicken breasts covered with grill lid, over 350° to 400° (medium-high) heat 6 to 7 minutes on each side or until done. Pierce chicken with knife to make sure juices run clear.
Place tortillas in foil, and place on bread rack of grill to warm. Place on spinach leaves, drizzling Italian dressing over all.