what is the best overdrive transmission for a Duster

Any MoPar 4spd trans will jus bolt in and work. How ever, the gear ratios are what need to be addressed. Also realize that the OD gear is the weak link. Also, the one cave eat to it NOT being a bolt in is the bearing retainer size. You can get an idea if you go to Brewers web site where they have many parts for sale. You'll notice various bell housings with various sized bearing retainer holes for the various transmissions bearing retainer sizes.