Your calculations are garbage. You cannot spend coins 80 times in an hour. The attacker has the power to rewrite a history that doesn't include him spending the coins, that is all. He can't simultaneously convince 80 people that they have the same coins.
FreeMoney is absolutely right.
The only way to get 80 people to accept the same 400 bitcoins would be to control all of their bitcoin connections and feed them different versions of the block chain.
And THAT will be impossible, because the people you're trying to rip off (merchants selling stuff) are exactly the people with long-running, well-connected bitcoin nodes.