I like iron so I said iron, that being said aluminum or iron if a brand new the cc's and compression will be provided for you from the mfg, if an old undetermined head you don't know how much you can safely mill off without the piston striking. CC's are the way you determine how much has already been or not been milled so you can continue without damage. So if you don't know what the stock cc's were how can you figure out how much has been or not been milled already? Ma Mopar did not provide us with this measurement so we have a crap shoot milling iron heads with a dubious history.
Can you hear me now.....lol. I hope that helps.