Need D.S. 273 manifold or substitute

Yes, ding the oil pan for clearance, but don't use a metal Hammer, use a mallet or put a piece of wood against the pan and hit it with a metal hammer. You don't want to crack or put a hole in your oil pan.
Also, I have a couple of driver side exhaust manifolds for the early a bodies. On other cars I have had I have match ported them and smooth them out and they seem to run pretty good.