It's the G56. They offered them for only a year or two, behind the Hemi in the 2500. There was actually a ram 2500 (with powerwagon badges) just for sale at my local dealer. I ran the VIN on Ram's website, and per the factory sheet, it said it had the 6 spd G56 in it. It is mildly rare to find one, and they're geared (as far as I know), similar to the Cummins. I would love to have one, but can't afford to do the swap, nor do I have the know-how.
However, someone with speed in mind would be better off with the TR6060 (IIRC), which is the one out of the Challengers. There's a company that makes a custom bell housings to make all the magic happen so it fits in the Rams, or so I've heard.