best cruising gear ratio 440 Duster

What's the rest of the combo ??

As long as it's fairly streetable engine you should be.

But depends what gear your coming from how performance you'll lose.
If you mean drive train, I have 727/8-3/4. I have just acquired the car. It seems to be running about a 3.43 rear end right now. 2000 RPM @45 MPH, 27" tire. Previous owner was in poor health, so I don't have every bit of data on the car. 440 is a recent rebuild.