GCWR is not really the limiting factor on a TH400, it's the torque being transmitted through the clutch plates. That's a function of the engine torque, rear end gear ratio, and tire diameter as much as the GCWR. You'll be able to move a much higher GCWR with 4.56 gears as opposed to 2.73s - you just won't be able to go as fast.
And which version of the TH400? There are a bunch of versions with different numbers of clutch plates in the clutch packs.