I'm not sure about the differences you questioned between the 350/450's construction but I think the 350's are just a tad narrower but... don't sell the E-350 short. In a 24' C the E-350 is definitely the way to go. Our 2012 Nexus (see sig), which has the 2011 E-350 chassis, rides and drives very well. Our 350 has a 4:10 rear end vs the 450 which is much lower (4:53 I think). This gives better mileage and since the suspension is softer you get a much nicer ride with out sacrificing handling. In 35 mph winds it is still one handed steering, tracks straight as an arrow and 18 wheelers hold no terror regardless of their speed.
Look at OCCC when shopping as a lot of C's are woefully short in this critical rating. Our E-350 has 3,368# of OCCC and I have yet to encounter any 450 that can top that. The 450's are heavier and that eats into your payload. Our GVWR is 11,500#(the newer models are 12,500), and fully loaded for travel we weigh right at 10,050 leaving an additional 1450# of available payload.
Also make sure to get the larger, often optional, 55 gallon fuel tank as this will give you much greater range than the standard 40 gallon. It is nice to be able to travel 400 miles and still have nearly a quarter of a tank remaining. You will want the 6.8L V-10 which is one very reliable beast and matched with the 5 speed Torque Shift transmission is a formidable combination. You can expect about 9.5 for average mpg on just about any road.
:C