Just too big and certainly to heavy for the 3.5 GVW vehicle segment.
An aluminium quadcon or tricon is not necessarily that heavy.
Quadcon has an empty weight of a few hundred kilos, but a Transit Chassis Cab (Cutawy is apparently the American term) is a couple hundred kilos lighter than the regular van, so the weight disbenefit is not that big.
EDIT:
Transit 350 L2 Chassis Cab apparently has a minimum kerb mass of 1619kg
Transit 350 L2H2 has a minimum kerb mass of 2054kg.
So we save something like 435kg.
So it gives you some weight to play with for a container system.
EDIT #2:
L2H2 has a load capacity of about 10 cubic metres, comparable to a Tricon container.
Tricon container masses down to about 1170kg in Corten steel apparently.
So on the face of it you lose about 700kg of payload mass, but you can the flexibility I mentioned, I wonder how many delivery companies are mass rather than volume limited in normal operation?
Also worth notign that a Tricon, thanks to being a box and taller than a regular transit body (more like a Luton van) could allow a short wheel base design to be built which would get more of the weight back, and also allow shorter turning circles.