Yeah you're right and I'm aware of spring changes. However for my case of a 24 ft. Class C, only an E350 chassis is required. When we bought it new, I had looked far and wide to find one on the heavier duty E450 chassis, which I finally found at a dealer and bought it. I wanted the E450's more rugged running gear and lower differential gear ratio so we could take our small motorhome offroad occasionally.
So in effect, our stock E450 leaf springs and fronts coils are beefed up from what would have been the adequate ones on an E350 chassis.
The Ford Econoline E-Series van chassis leaves generous room in the wheel wells for larger tires than what typically comes stock on them. The best way to gain more ground clearance everywhere for the ground side components of a vehicle is by fitting larger diameter tires on it instead of lifting only the structure above the axles ... that's why one of my priorities was to put larger diameter tires on our Class C MH.