I have done it many times leaving the cast iron manifolds attached to the front pipes. The method is a little more technically difficult and is best left for the experienced professionals. Try to use the prefered procedure and finish disconnecting the front pipes. Leave the manifolds bolted to the head for now. Gentle taps with a lead or copper mallet can sometimes free the manifold/frt. pipe connection. Do not use a lot of force with a steel hammer or hard steel mallet or the cast iron manifold can break. In severe cases unfasten all exhaust hangars so the exhaust hangs freely with some support from a stand or jack. Grab the exhaust pipes at the tail pipes and move system by hand, back and forth and up and down. Another set of hands can tap the connection with a lead or copper mallet at the same time if necessary. This method almost always works. Another technique is to use a torch to heat the connection if the equipment is available. Once the connection is free you can unbolt the manifolds from the cylinder head and remove them to continue.
If you are still not able to succeed, let me know, and I can fill you in on the alternate procedure.