Load an E-350 and a 450 to their GVWR's {as many of us do} and the 350 will be between 2,000 and 3,000# lighter {depending on the model year} yet both will have the same engine and trans. The small difference in empty curb weight between them illustrates the minimal differences in the construction of the two chassis'.
Payload, much like towing capacity is more a matter of marketing than actual physics.
As far as the difference between the 362 HP V-10's found in Class A's those are the 3 valve model that are too large to fit into a Class C application.
:C
