Open? .... Posi?

Before you swap it, take a backlash reading on the 489 housing/gearset. When you put the sure grip in, reset it to that backlash setting.

As Kimmer mentioned, the ring gear bolts are left hand thread.

So you don't have to beat the ring gear to install, make sure the wife isn't home, turn on oven to about 200* and warm up the ring gear for about 10 minutes. It will drop right on and start the bolts, don't go crazy tightening them, just get them started.