So, the NV4500 lost 5th gear. I'd like an extra gear, and looked at my options.
The NV5600, which came in these trucks 99-02, and 03-04 would be the simplest. However like the NV4500, overdrive is on the back of the trans, causing a weak point. Plus, the 5600 is getting hard to find, and most trans shops won't touch them, citing many issues.
So I started looking at the G56 used from 2005 to now. It is a German design, and the reviews out there pretty much say its God's gift to the manual trans Cummins crowd.
There are two G56 versions, the 2005-08, and the 2009-current G56a. The difference is that the early model uses a .79 overdrive ratio and the current model uses a .74 overdrive ratio. There is also rumor that the mainshaft and some internal parts were made of better steel (cryo-treated?) in the newer models, although I have yet to positively confirm this.
So, after thinking about it and checking the numbers I've decided to use a 2009-now G56a.