My opinion here.
We own a 2007 DRW cut-away E350 (11,500 GVR) class B+ rig, the last year of the previous vintage chassis made prior to the current-day E-Series. I believe the single worst culprit with the 2007 E350 & older chassis was that straight from Ford, they were NOT equipped with a rear stabilizer bar. If you install a heavy duty rear bar, you resolve the worst of the handling issues, making it real close to the current-day E350. The improvements introduced for 2008 for both E350 & E450 tweaked the length of the upper & lower front control arms along with higher rated springs which also helped, but on the E350, that is much less influential compared to the rear stabilizer bar.
The Ford installed stabilizer bars regardless of vintage are not as effective as the heavy duty aftermarket bars from Roadmaster and Helwig.