Mine is an E450; There are a several companies that make an after-market cat for an E350 with the same engine. I contacted them and got roughly the same response from each:

> Unfortunately we do not have a catalytic converter for your vehicle. Although it may seem that the vehicles are similar, in reality they could be completely different when it comes to emissions. We've seen some cars that are the same year, same make, and even engine size, that use completely different catalytic converters. It has to do with the manufacturer and which emissions tier they are registering that vehicle for. That being said, although the E-350 maybe similar to the E-450, in reality the converters and emissions level could be completely different and that's probably why we don't have the certification for it. Sorry I couldn't help you out. Take care.