8 3/4 741 or 489 case are they interchangeable ?

I don't know the difference between the 2 cases but I know I have swapped the 2 out before.

I have a 2.76 single 741 case in the car now
and a not so limited slip 489 case with 3.55's in it sitting on the bench.

I am going to send the 489 to Dr. Diff to rebuild and then put it in the car.

Are they interchangeable and what is the difference ?

thanks for the help.

David