Dart's have a 111" wheelbase, Dusters have a 108" wheelbase. So as Joe pointed out that's exactly the 3" difference. It's not the tank, it's just the wheelbase of the car.
Not using the stock tank is a pretty big drawback, and I don't even care about not being stock. I'm not a big fan of having the fuel inside the car, regardless of it being in a fuel cell. And the loss of trunk space is a problem. That's actually the biggest issue for me, I use my trunk because I use my car as my daily driver. And since it is a driver I would need a large fuel cell too, anything much less than 16 gallons would be a PITA on long drives.
Still interested in seeing the rest of the installation. I would love to get a measurement from the axle to the back side of the new crossmember though.