This thread is a collection of information I have found regarding the installation of the 722.6 transmission in a W460.
After much consideration, I've chosen a 5-speed auto over the Getrag 5-speed manual to be used in my 606a swap this winter. I'm certainly not the first to have the idea to use the 722.6 in a G but there's much to learn so I thought I'd post some of my findings in case there are any others out there interested in the same thing:
First, if you are wondering why the 722.6, it's an extremely tough transmission made by MB and used in many other vehicles such as Land Rover, Porsche, Chrysler, and others. The 722.6 is also capable (if the right version) of handling massive amounts of tourque (it's even used in the V12s) - something the manual transmissions lack, at least "officially".
I'm assuming that most G owners would only want to consider the 5-speed auto if they are also installing a 606 or 606a diesel. Most would say that this is the diesel of choice for USA-based owners since many of the alternatives were never offered here. As such, you must select a transmission from an M104 or M111 engined vehicle. These will have the correct bell housing to mate to the 606. One added benefit of this is the transmissions from most 4 and 6 cylinder vehicles will have lower gearing in 1st and 2nd gears - better for off-road conditions, and still great around town considering the weight of the G. Transmissions from the larger-engined vehicles will have taller gearing which will give you sluggish performance at low speeds.
It should also be noted that the 722.6 has gone through a long process of improvement since it was introduced in 1996. As such, the newer the transmission, the better it will be. Obviously, try to get a low-mileage, late-model unit that has it's original torque converter included. Most notably, the early transmissions use bronze bushings that were later replaced with caged needle bearings. The bushings have been known to wear and cause misalignment of the planetary gears.
Once you have your transmission with the correct bell-housing and gearing, you need to evaluate its condition. If it is a late model, low mileage trans, you may be able to simply use it as-is. If not, it is a fairly easy transmission to overhaul yourself, implementing some upgrades as you go. The most common weakness is the K2 clutch pack. Smaller engined vehicles use fewer clutches, and of a smaller diameter in K2, translating to shorter life and lower torque handling. It is possible to replace the clutches, and add up to a total of 5 (I think), and I am in the process of trying to find out if the K2 assembly can be swapped for the larger diameter version used in "heavy-duty" applications.
Other basic considerations are the output flange (needs to be swapped for G type), short prop shaft (need one of the right length - probably custom but I'm not sure yet), trans mount (looks like new holes need to be drilled but not too big a deal), and most importantly the TCU. There are 3 TCU providers that I have found, all with their pros and cons. PCS appears to be a US based company that makes one TCU for use in various transmissions through configuration. The whole shebang costs around $1000 and does not officially support the 722.6, though the Seattle dealer, Sound German, offers the needed firmware. Another option, the one I'm going with, is made by Ole in Europe. The whole kit costs 650 Euros, including all options, and sensors (4-3-2-1 type shifter and TPS provided by you). One major plus is the TCU comes with its own LCD & joystick that you install in the vehicle. It can be used to configure and troubleshoot in the field, which gives some significant peace of mind.
Okay, that's as far as I got so far. I hope to add to this as the project evolves. If anyone out there has tips to help me along, please pipe in!