what is the best overdrive transmission for a Duster

The A833 OD is not a 3 speed. I don't know why people insist on calling it that. Anything with the A833 designation is a 4 speed transmission. The 833 OD just happens to have 4th as an OD.

The best one for you car is relatively rare. It is the 1976 only transmission, which had a cast iron case. Find one of those, if you can.

To get an A500 or A518 in your car, you will have to do some major work to the floor and to the torsion bar cross member and the transmission cross member. If you want to stay automatic, the GM 200R4 is probably the best.