From what I have heard removing the manifolds from these 6 cylinder heads often leads to the discovery of non-planar surfaces requiring machining to correct flatness for any hope of proper sealing after they are installed. I do not know how correct this is. I would think that if an engine is properly taken care of and does not ever overheat this is much less likely. However, I did not design the manifolds so, it may be best to err on the side of caution.
I have removed several freeze plugs in different engines using a hammer and chisel. Keep in mind you want the freese plug to rotate out toward you as much as possible. I still found it required a good pair of lock jaw type pliers and heavy pulling to remove them but, I was able to remove them without removing manifolds.