I have two suggestions on how to remove the head from the barrel. I am assuming that the liner has stayed within the barrel. Put the whole assembly in a large container of water, bring to the boil and simmer for a while. Alternatively, get a piece of wood, 3 x3 x12 inches long. Round the corners of one of the ends so that the end is curved a little more than the inside of the head. You are not trying to make the wood a good fit, just ensuring that it preferentially touches the centre of the head in between the valve heads. Then if you have firmly mounted vice, put the wood into that, long way vertically. Put the head and barrel over the wood and then, holding the parts by the outside of the barrel fins, lift it up and let gravity bring it down on the wood. Use more force if needed. If you use a bit of soft pine there should be no damage to the head but you might want to get someone or something to ensure that when the parts separate the head does not fall off an hit the floor.