Battleship 120? Come on, be serious! Whether or not you think that's realistic, it would suck for gameplay. It would make the unit invincible to anything except another battleship.
No, I think leave things the way they are, with the possible exception of dropping the destroyer strength a bit, or increasing the ironclad strength. We don't need any 120 strength units out there!
NUKE! NUKE! NUKE!