From the 1970's through most of the 1990's, the E350 was Ford's only Class C chassis offering. Then in 1997, they went from 460 to V10, and offered the E-Super Duty. That is the beginning of the E450. After a few years, Ford dropped the E-Super Duty and branded their Class C chassis as E350 Super Duty and E450 Super Duty.
Maybe oversimplified, but
1992-1996 E350 with 460 and E4OD transmission
1997-8-9 E350 and E-Super Duty with V10 at 275HP and 4R100 transmission
2000-1-2-3-4 E350 SD and E450 SD with V10 at 305HP and 4R100
2005? -6-7 Same but with TorqShift 5-speed transmission
2008-15 Same with new styling
2016 Same with new Six-Speed transmission