Unsolved Add retarder to power train

I'd wager a simple shaft set to -1 as a gear ratio and able to be connected/disconnected to the engine may work with some rearranging of the powertrain, though you run the risk of stalling the engine or outright running it backwards at worst. Probably easier to implement than a rangebox. Viscous couplings rather than torque converters may be the way to go here, since the torque multiplication doesn't work in reverse AFAIK.