"Assume your PH will be of minimum assistance backing you up and you have to finish anything you start."

With this in mind, I'd say either the 450NE or bigger. But then I've only shot 1 elephant, and that was with a 375H&H. However, I was still able to finish what I started myself, with no interference from the PH. He was mostly occupied with keeping an eye on other nearby elephant anyway. But next time, I think I'll use something bigger, to be on the safe side, and because I don't blindly trust that anyone else will take care of my safety but myself.

Erik