Not sure on the T-Case.
I have driven a medium-duty truck, a 1981 single axle IH dump truck (Ex ILL State Truck) with a 3208 n/a and an Allison Automatic. Is not bad, it had alot of low-end grunt. Truck was great to plow snow with. But it was not a speed demon. I think that truck is still In-Service.
One other funny thing about that truck, which has nothing to do with the engine, but the accelerator pedal got much stiffer and harder to push at speeds above 45mph. I do not think before we bought it it had ever been over 45....... I know it will do 65-70, with the mud flaps flying back....... It was still orange, and looked just like the ones that were still in service. If you went throught an area with "state guys" you got a really funny look if you were clipping along. I think most thought something was wrong or had happened.