Is it possible to put the Vortec V6 into an extended WB Express Van?
I know they don't come that way, but could it be done?

 

The engine would have to work much harder, so there would be no benefit. In fact, the larger V8, in reality, offers about the same mileage if you keep your foot light. You can even get better mileage with the larger V8 if you get one of those re-programmers. But as discussed in other threads, those can void the warranty.