The primary difference is in the diameter of the pinion gear shaft. If forget the exact dimensions, but it goes something like this; the 751 case is 1 5/8", the 742 is 1 3/4". Both use a flat washer to set pinion depth so they are perceived as being able to withstand a greater shock load. The 489 case uses a 1 7/8" diamter pinion and a tapered crush sleeve to set depth. The taperd sleeve makes it easier to set up since you don't have to disassembled it to replace washers while setting preload, but high rpm launches can also slowly crush the sleeve and eventually lead to the pinion depth being incorrect, whoch could lead to a rear end failure. There are solid spacers available for 489 to use in place of the crush sleeve. Gear selections for 751 cases is limited to what ever you can find on Ebay as no one makes gears for them anymore. For a mild street applciation, mild being up to 400 horse, any of them would work fine.
