Driveshaft length

Do not add anything, subtract. The drive shaft must be shorter than u joint centers. Exactly what car and drivetrain are we talking about?

Ok the car is a 74 duster that had a sb360 with a 727 and an 8.75 Rearend . It now has a 4 speed with the same rear end I'm going to use the same yoke from the 727 shaft. Now I was told the a833 is 4 inches shorter than a 727 . How do I get the correct length.