Hi Richard,
The only thing holding the barrel should be the 6 base nuts - plus, of course, sheer stubbornness!! Probably some form of gasket cement has been used in the past, which has tried to weld the barrel on. You could try replacing the head, and then with the base nuts undone about 2 turns, try kicking the engine over - the compression may have the required effect and start lifting the barrel. Once the barrel has started to move, it should come off quite easily.
Regards,
Eddie.