I bought Husky floor liners for my E450 10 years ago; the only 'vehicle' listed was the 'Econoline' series, but a call to the vendor (I don't recall the one that I used) assured me that the liners for the E350 would fit the E450. They did.
I recommend buying for Auto Anything; they have a wide selection, pretty good prices, and good customer service. I suggest calling and asking any 'fit' questions. The liners are not cheap, but, as I said, mine have been in the rig for 10 years and show very little wear.