5 speed options

For that kind of power a T-56 (6 speed) or TKO 600. The TKX was just released and TKO kits might work for it. You can get both cast and blow proof bell for any of the 3 (as long as the TKX uses all the TKO mounting locations) and you can reuse your Z bar or but a Hydraulic system.
Unmodified T-56 and TKO will need the tunnel widened.

The T5 is too weak for what you have, even a mild built 360 and good traction would tear one up.