Looking at the Eaton transmission catalog it would have to be an 11, 13, or 18 speed to get a better overdrive ratio. All the other models run a .78 overdrive, the ones listed run a .73. I was planning on upgrading to a better overdrive anyways.
I'd change trans if necessary to handle the torque. From what I've read here, seems like 450-500ft/lbs of torque is about max for its stock reliability. Please correct me if I'm wrong. Would save money if the stock trans could be reused.
I use the t-case currently as a gear splitter, works great. What is its torque rating? I know an np205 was used in the 5 ton dump trucks and is much shorter, if a swap is necessary. Lose the synchronized tcase though.