The picture above is the carrier for a late model Bronco. It was sturdy enough for dirt roads, and could probably work well for your application. You will need to reinforce the mounting areas for all the brackets so it doesn't tear the sheet metal apart, but I think you could make it work. The hard part will be making those brackets work since there were contoured to a Bronco body and not an E350. The upside is they are plenty strong enough to hold up to a 33" tire with no issues, provided you have that reinforcing on the body I mentioned.