News:
Please don't take offense if this is a stupid question, but is your plan to put the posi unit into the 489 case, or rebuild the 741? I am assuming you are re-using the 489. With that said, are you sure the 3.91 gear set is for the correct carrier?
I had the same problem with the pinion nut on another build I did, are there any 4x4 or diff shops in your area? they should be able to get (or have on hand) a pinion nut