8 3/4 Cases

741 = 1 3/8" straight pinion shaft - pinion bearing preload set with shims

742 = 1 3/4" straight pinion shaft - pinion bearing preload set with shims

489 = 1 7/8" taperd pinion shaft - pinion bearing preload set with a crush sleeve

All use the same carrier bearing size and carriers are interchangeable.

The aluminum MP case is a 742 based case. The 489 doesn't really offer any strength improvement over the 742 case but the shim set pinion bearing preload of the 742 case does allow you to remove the yoke without having to worry about replacing the crush sleeve. This setup also means you shouldn't have to worry about the pinion nut loosening up from the crush sleeve losing it's tension.

Parts availability for pinion seals is not as good for the 742 case though and there is a conversion available from Ratech I believe to convert the 489 case to a shim set up for pinion bearing preload.

So in summary if you have a 742 case I wouldn't worry about finding a 489 case and vise versa as most gear set ratios are readily available for each.