Air gap makes more power, heat in the manifold makes them so much nicer to drive. One thing I will never do again is try to run an auto trans without a heat riser, it is very anoying! Next issue is sound! If you run without a heat riser I would want an "X" pipe or some other cross over balance conection in the dual exhaust. Without a balance pipe in the exhaust or a heat riser cross over the exhaust sound is ugly! I do not mean ugly loud I mean braaaaaaat ugly! I recently did a 454 conversion from 350 on my 72 and bought a no heat riser manifold used. My neighboors would plug there ears when I started it and it was not mocho, it was ugly! Put a heat riser manifold on it and the car is a joy to own.