Can someone please explain to me the differences between all the cases available for 8 3/4 rear ends?

 

well there is the 741 case the 742 and 489 case . it goes strong,stronger and strongest in the order I listed. There are other earlier cases with other numbers but they are virtually the same as the 741. The 489 case has a tapered pinion that is larger. The 742 is the most common used in the high perf cars of the late sixties. the 489 I think came out in 69 but not sure. It is stronger but less easy to build {slightly} and parts not as easy to find [somewhat].

 

what case would an 67 automatic 440 have gotten?
could it have gotten the 741?

 

Not sure. Look at the casting numbers on the driver side of the case the last 3 digits delineate which case it is.