what is the best overdrive transmission for a Duster

A Body 833 transmissions are not that hard. It really does not matter what gear ratio it has. The O/D gears are no big deal. A body tail shafts come in 26 spline, 30 spline, and ball and trunion outputs. A Body O/D tail shafts come in 30 spline, same as standard A Body, and 26 spline different from the standard A Body. You can swap the standard main housings on any A or F body to the O/D and tighten everything up and it will slip into any bellhousing. The only difference then is the 3-4 shifter rod and arm. The earlier the O/D the better the gears. If you have a truck O/D, I'd change the main and tail housings and main shaft. They also are not as weak as one would think.