Grigg is spot on with the ebrake applications. I've only seen one for sale that has the ebrake on it, but they are out there.
Those fuller 6 speeds came in all kinds of trucks. A lot of guys will buy a used '90's fl70 (freight liner) just for the drivetrain (cummins 12 valve and fuller 6 speed). Otherwise, hit the medium duty junkyards.
ZF makes the zf s6-650 and s6-750 (zf6 for short). These were used in ford trucks, but they were also used behind the duramax and the chevy 8.1 (496 big block) until 2007. The chevy versions are more rare, but they can be found. A chevy zf6 would bolt right up to your motor and only weighs around 230lbs.
The g56 is a Mercedes transmission that Dodge has been putting behind the cummins in their trucks since 2005. It weighs about the same as the zf6 and is constructed similarly as well.
Easiest swaps will be nv4500 and Chevy zf6 since they will both bolt up to your small block without adapters.
The main advantage of a 6 speed is that you don't have the bad 3-4 split that you see in all the 4 speeds as well as the nv4500. It's not a real bad split unless you are really hauling a lot of weight.
