Husky does not list having floor liners for the E450, but they do for the E350. I had assumed they are identical but am I wrong? I don't want the hassle of returning them if they don't fit.
The cab is the same across all the Econoline series, floor mats for a E-150 would work if you are looking at seat forward.
The two cutaways (E-350 and E-450) have the same cab length from the factory, but different upfitters may change that dimension, by adding a rear extension between cab and box.
I had a 2011 350 and traded on a 2014 450 and everything cab wise is the same. The chassi I think is the only difference, 450 is heavier. That is my observation anyway.
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.