Well good luck a 12 bolt, since they have found a value in the racing world, they are very hard to come by.
I would go with the Auburn. The price sounds a bit high, but if you want to have a pretty soild set up. Sounds like it would be good for racing, but if you're not racing the car, or it's seeing rare track use, maybe the currect rear is fine
3.08 would be the cheapest way to go, since you can use the same carrier. 3.23 and up use a 3 series carrier