If you have to make a rule that these weapons can attack land targets then you do this:
roll 4 dice total = ipc in pieces lost.
example: you roll 18 total on 4 dice, and 6 infantry are gone.
This makes it more value to use against production targets, because you get one die that has lasting effects.
I like the strategic bombing rules
but although atomic weapons are not the perfect ship killer, they would be much better against ships than ground units, if each ground unit represents an army then even four atomic weapons are going to have a pretty hard time of even destorying one of them.
What i suggest is a heavy bombers can choose to drop it in any combat round. The bomber rolls four dice and they hit on a four up. However after you drop a bomb in a land territory the battle has to immeditly stop and after the defender returns fire, the attacker must retreat unless he has killed all the defenders.
This is close to history and add strategy, what more do you want?