Our old 440 van gets around 10 mpg on the road and about the same towing my son's Challenger on an open trailer, 391 posi rear.

There are lots of danas out of pickups if you want a stronger rear end. Width would probably be in the ball park,

There is a lot of room under the van for turbos, use alcohol injection and you have beast. Better get a better drive shaft too.
you could add a gear vendors overdrive.

Also, these old vans steer like an old mopar...plenty of slop in the wheel.

My son still has my old 77 van and they are a very practical vehicle. We can move a lot of things without worrying if it will rain. Being in the northwest rain in a big factor.