IMHO the AC will feel more efficient if the vent placement allows cold air to hit you in the face. I have a 55.1 with a GMC dash. I've got a center vent and two corner vents under the dash. It cools well but it takes a while to feel good. I think moving the center vent up higher would help. The speaker grill vent should get that done in the Chevy dash.
regards,
Leon
