Several of the 350/450 differences have gone away. 350 got a wider rear stance not long ago and also hydroboost brakes. 350 didn't have rear disc brakes but now does. Also now has rear sway bar like 450 always had. To me that's no big whoop since all of Ford's OEM bars are inadequate for an RV anyhow. 350 can also have a 5000-lb capacity front axle. So, in late models, yes, 450 has the huskier frame, but about the only difference most of us would ever notice is rear axle capacity.