F350 Srw
The 2008 F350 is alot different than the 2004, different engine, different suspension components, different wheels, different weight ratings etc.
I have pulled through Colorado, Nebraska, Iowa, Kansas, Oklahoma, Missouri, and South Dakota. The grades have been steep.
I have towed with various vehicles and honestly if you ever feel that you have nothing to worry about while towing you are overconfident. You will always take more distance to stop, handle differently, react to wind and grades differently and should always be more alert.
I have friends that tow with dually trucks for hauling cattle, liquids, and other mobile rolling stock. The additional width of the rear stance can give you more rocking resistance when you are pulling a trailer. The additional wheel also gives you a higher payload. That said, dually's don't allow you to stop faster (you still only have one set of brakes, not one and a half) and the physics of the weight distribution for a fifth wheel don't improve the stability as much as you might think.
The F450 is not an F350 with duallys. It is a different frame. You have a great truck! But I can tell you that the 2008 F350 SRW is also a great tow vehicle and has proven itself many times over.