Looking for a 8-3/4 third member with sure grip and 3.55 gears. Must be either a 742 case or 489.

 

You know you could buy the ring and pinion (gear ratio) you want then buy the locker you want and have a shop swap it all into your center chunk (from your 8.75 rear end)
It's an option,
But,
EBag is a good place to score complete third members but watch the shipping cost and look over the pics REALLY well.