What length of transmission do I need for 71 duster?

Any A body trans will fit but with that rear you are probably going to need a custom length driveshaft no matter what trans you go with. A 3 and 4 speed will be the same overall length so that won't be an issue. 66 and up manuals use a splined drive shaft 65 and down use a ball and trunion so stay away from those.