Froch is too strong and young for Johnson. I like Froch in a very good fight where Johnson just can't keep up and, in the late rounds, probably takes his worst beating since the Hopkins fight all those years ago.
The UNDER at +250ish is tempting, but even into his 40's Johnson has showed that IRON chin he's famous for. Only chance at a stoppage would be by the corner... I would lay of it.