what is the best overdrive transmission for a Duster

Is your car a manual Or automatic? You didn't say.

The manual OD 833 can be picked up for a reasonable price used or rebuilt from brewers at a price.

The 904 was given a OD unit on the tail shaft and called the A-500.
It is leaner in weight and size next to the 727 OD unit called a A-518. It will fit a little easier. But it isn't a slam in and go thing.

You'll need to shorten the drive shaft
Depending on what you have now and install later, the trans yoke is a consideration. The manual and the 518 use the same home while the 500 uses a different one.

In any case, there is not to much improvement with a OD gear set if the engine is a built up one. The fuel consumption could largely be due to a large cam, carb and other items.