I reckon if Johnson does not KO Joe early on (first 6) then he wont win. Whether Calzaghe hurts him or not is beside the point as he will score more punches which should win him rounds. As long as he doesn't get rocked early on by Johnson and playing catch up.

Other things to take into consideration:

For Joe:
It is his home stadium so BIG pressure
Joes Hand
He has been training for 168 and its now probs going to be catch-weight

For Glen:
There is not going to be much support for him
He needs to drop weight quite quickly

I think that Glen will just push it and sweat off some extra pounds to make the catch-weight without actually losing much more actual weight, meaning in the ring he will be significantly bigger than Joe. However taking everything into consideration ithink that the home advantage will see Joe right, and he will win by UD, and that it is an excellent stepping stone to to LH should he decide to go that way