The tail end of the Ultravan is much wider than the Dymaxion.
However, the Dymaxion only seats four, while the Ultravan has much more interior room, enough to live in.
The Ultravans are relatively inexpensive and could be the base for an interesting ecomodmobile. However, they only come with Corvair engines and Chevy 350 V-8's, neither of which is a very good engine for fuel efficiency.
If one were to find an older Ultravan in rough shape, strip out and rebuild the interior to make it as light as possible, clean up the exterior and repower with a better motor, what type of mileage do you think it might get?