I think you're estimates are perfectly fine, and if anything, they'll likely be on the conservative side. I suspect you might be able to make slightly better time than you are expecting, at least once you get comfortable on the road. I'd expect that first day to go pretty slow as you get used to driving the large truck/trailer combo, but that will get easier as you make your way on the road.

I would very much agree about getting a diesel if possible. I've used Uhaul for my big moves, and diesel will get you a few extra mpgs and seems to have a bit more power than a gas engine, especially when you'll be dealing with one of the larger sizes of trucks.