"In 2008, the E350 chassis GVWR was increased to 12,500 pounds which stands to this day for the 2014 model year. "
Sorry but that simply is not true. My 2012 Nexus sits on a 2011 E-350 chassis with a GVWR of 11,500#. The 2012 and newer E-350's have been bumped up to 12,500. Even at 11,500 mine comes with 3,368# of payload. Fully loaded for the road we weigh 10,050 leaving me an additional 1,450# available that I will probably never use.
The 450's have more GVWR, I think they come in at 14,500 and a 4:56 rear end vs the 4:10's on the 350. You will get a rougher ride and worse mileage with the 450 but if there is a Toad in your equation the 450 is probably the better choice.
IMHO: Don't sell the 350's short. On C's 24' or less the 350 is the way to go, better of ride quality/handling and mileage yet still come with more than enough payload.
As always.... Opinions and YMMV.
:C