Don't think the fight would happen, and I think Silva, like everyone else, would have trouble with the reach. But, I think his wrestling has improved enough that he would be able to defend against the takedowns fairly well. I think the fight would be a lot more competitive then people are saying.
However, I'm not sure GSP would be competitive at all anymore. He's looked kind of flat in recent performances and has been unable to train due to injuries for a long time now. Meanwhile, Silva has improved drastically.