I've seen a lot of problems with ring gear spacers. They move the ring gear away from the flange on the differential where there is a machined sholder that centers teh ring gear. This causes the ring gear to float on the differential and only the bolts hold it in place. This causes contact pattern problems. I have seen several 12 bolt rear ends that sheared the ring gear bolts off because of a ring gear spacer. Ring gear spacers are a bad idea and should be avoided.
You can get a 3 series Eaton Posi case and transfer all of the parts from your 2 series Eaton Posi into the 3 series case.