Jim, Let’s make a few assumptions here: 1) The head gasket has a copper surface in contact with the aluminum head and another copper contact with the cast iron block. 2) This assembly has probably been together for about 90 years in a hot environment. 3) No head gasket is perfectly tight, so some (conductive) coolant or exhaust gasses was between the gasket surfaces.
If these are true, you are now into the high-tech world of bi-metallic bonding. Search for “bimetal bonding” bonding between Aluminum/Copper and Copper/Iron surfaces bonds are known, some with commercial uses such as thermostats. After reading, don’t expect the Aluminum/Copper bond to break at all without machining the gasket away. Try to split between the copper gasket and the iron block. If the gasket has a compressible asbestos core, you may be able to cut it with a very thin hacksaw blade.
There is a discussion on reducing the Copper/Iron bond, but that chemistry is beyond me. Check the “BBC.UK/Bitesize” site for a good discussion of this topic, including suggestions on reversing the process. Regards, Herb