I'm in the middle of doing this same swap in my 73 W200 which also has a divorced transfer case. To deal with the extra length issue I ended up using a 2WD chevy NV4500 with a dodge input shaft swapped in. The chevy tailshaft is way shorter than the dodge one. I need to shorten my driveshaft a bit, but dramatically less than what you are talking about.

It might be less effort for you than moving the T-case and making all new driveshafts and mounts.

-Jon