The E350 is just basically an elongated Van Chassis with a Ladder type frame. The frame is made of C channel with the front end "boxed". The diff between the 350 and the 450 is significant. My neighbour has a 450 "super-duty" that he uses for hauling junk. It has front leaf springs (usually this happens on vehicles that handle more weight) and the frame is the same style but "beefed up"... it's larger... but still the same C channel frame. The difference in the GVRW is more spread out then I would have thought. The GVRW on mine is 4500kg (10000 lbs).... his is 8700kg (not sure what that is in pounds... x by 2.2) now that's a big difference if you wanna add basement storage and such... my bus BTW is a 16 pass so your GVRW and mine should be close.
On my last conversion when I was almost done I had weighed it, and it came out to 4400kg or so... I figured that I was pushing it too close and decided to strip her down and restart. You see... I had already blown 2 rear tires and wasn't ready to pay for a 3rd, this is the main problem that you will run into when you overload it... tires first, bad handling... and then who knows... you may just "Pop a Leaf"... now that just wouldn't be fun driving down the Highway.
I figure when my bus is near completion I am going to "re-arc" the rear leafs or add on a set of "helper leafs"... Kinda like what the super-duty's have... plus I will be welding on mounts for another set of shock absorbers which should improve the handling under weight... (but you still do not wanna overload this) remember tires blow first... this is just for road handling... another consideration that my big rig driving buddy suggested is adding an "air ride" system to the back... it really makes the back feel solid, and the road handling is great.
If you do wanna handle MORE weight in your rig... well boxing the whole frame would be an option... but then you would have to change out the rear end.. change the front end... pretty much make a new chassis... you would probably be better off getting the proper sized rig with a higher GVRW... rather then intensely modifying yours because after all the work that you would have done to it... you would still have to get it Inspected by DOT or something to confirm that yes... you can now handle more weight.... the GVRW... not something to be taken Lightly
Ha... made a funny.