Things like that don't always happen -- certainly not overnight. Colonies that become free of the mother country and its regulations often don't have the resources to make good once they became independent. Also, slaveowners' desire for a highly controlled society cut into industry and development and that wouldn't change with a drop in the tariff.
You forget. They were "rich". Some of the animosity I think many northerners had for the South was the result of this wealthy "aristocracy" making all that money off of free labor.
With 200 million in exports versus the North's 78 million, i'm pretty sure they had the capital to expand their facilities to handle whatever traffic was necessary.