A833 driveshaft length?

From the 1971 FSM.......

Since the Duster never came with a 440, I took the differences and compared them from the same swap, but using 340 as a baseline......

It looks like the 833 shaft needs to be 3.97 inches longer than the 727 shaft......Assuming the shaft that you are using now is adequate for the 440/727 combo.