OM648 swap into a w123
OM648 swap into a w123
I would stick with the 722.6 that comes with the engine. There is a standalone that works well with it and it makes up to the Rubicon transfer case for a very low 4x.
The 6r80/6l80 is also a choice and maybe a little more attainable since they are currently being used behind a number of current vehicles. Bob bendsten makes adapters for the i6 to 6r80 and I believe the 6l80 as well. It may be worth a phone call to speedgems to see.
Im currently in a similar swap and will not be wasting my time on any of the 4 speeds because the gearing is trash for a vehicle that will be off road, and after the cost of adapters and controllers you will already be at the same price of upgrading to newer tech. The zf6 that the 6r80/6l80 are based on comes with a 4.xx first gear and will give a perfect gear spread to keep your TD in the sweet spot on and off pavement.