what is the best overdrive transmission for a Duster

You'll save more money by tuning your carb and ignition.

I have same car, same engine, same tires, same axle, built 340.
-A-833, 750 double pumper, but tuned fairly well, 11-12 city, 13.5-14 or so mixed with some hwy, cruising at 3000
-A-833, Sequential multi port EFI, 13-14 city, 16 with a lot of longer mixed 45mph to hwy driving with long trips
-T56 magnum with .68 OD, multi port EFI, better first gear, 14.5-15 city, 45 driving has been over 17, but never went on long trips so far, but could probably get 20.

Any loose converter will make it worse. But it shouldn't be 9.6 mpg.

Years ago when I had the 318-2bbl, it got 13.5 pretty much constantly with a stock 904.

With a carb you're kind of at the mercy of the accellerator pump and it hurts city mileage, especially with a stick, with the EFI I have a lot more precise control. The EFI also goes into overrun when coasting so it injects no fuel in certain situations. But I fully believe you can get to the 11-12 mpg range very easily and probably on a trip cruising at 3000, 15-16.

Get a good wideband and a vacuum gauge and start tuning it.