I'm setting up a mopar diff for the first time. It has a new Aburn posi, Motive gear and kit. I've done 12 bolts and 9" before, is there anything special to watch for?

Not really....
What size pinion, 1 3/4" (742 case) or 1 7/8" (489 case)?
Polish the inner pinion bearing loc. to slip fit the bearing, so can easily set the pinion depth.

That's how I do the 9" ers, Thanks. The setup kit was short two ring gear bolts. Dang.
I haven't checked the numbers yet, I will.

Make sure you have the right carrier bearings. I think all the new kits though are made for the Auburn posi. I did a 489 case for my street car a couple summers ago, and used a factory Sure-Grip unit, which take a different carrier bearing than the Auburn units.

If you're doing a 489 case, I'd look into getting a solid spacer to replace the crush collar. I used the crush collar on mine, but if I had to do it again, I would have put the spacer in.