You probably want a NV4500 transmission. That's what comes behind the stock 5.9 Cummins now, after the Getrag was used a couple of years. They found out pretty quick that the Getrag was no match for the 5.9 Cummins, and switched to the NV4500. But be forwarned, its a VERY popular transmission, and it's hard to find one USED for less than $1500. New ones will set you back $2500. Advanced Adapters sells a kit to swap from the Getrag to the NV4500, and it includes the transmission. It ain't cheap, but it comes with everything you need. Here's their website:

http://www.advanceadapters.com/index515.html