Pull off the valve covers and get the cylinder head casting number. Well, only pulling one of is necessary. That will give you a lot more useful information than just the material type.
Although, they have to be iron. Aluminum does not rust like iron will. Aluminum is not a ferric material, so it will only oxidize. Basically get very dull.
Here's some SBC head casting numbers. http://www.kendrick-auto.com/chevrol..._number_re.htm
Hope I helped you out.
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