Torque doesn't affect the rudder and vertical stab; one isn't going to overtorque them.

Excess torque at low speeds does pose controllability issues in some aircraft, but this discussion is about cruise torque limitations or recommendations. Manufacturers pose "nominal" values for cruise as means of limiting long term thermal exposure for the engine, which helps limit creep and blade erosion.

The modern Air Tractor 802 has approximately the same weight and power as the P-51, but the P-51 experienced very different effects at low speed with rapid power application; that was a matter of a small rudder and an airplane designed to go fast, not slow. It was subject to a torque roll in which rapid power application resulted in yaw and roll in excess of available control authority to counteract. The solution; feed the power in more slowly. This was not, however a torque limitation, but a pilot technique. The 802, by comparison, at the weight and power, doesn't exhibit that characteristic.

The PT6A-67AG has a nominal cruise number of 720 degrees, but for climb, and at cruise, and at any time needed, it can be run indefinitely at 800 degrees, and often is when heavy.

Airframe manufacturers set torque limits for the structural integrity of the airframe, which includes the engine mount assembly and primarily the attach points at the firewall.

As an example, an airplane which has used radial engines with a power limitation may be retrofitted with a turbine engine capable of a much greater power output, but may be limited to the power of the original radial engine installation due to structural limitations. The engine can do more, but the airframe not necessarily so.

In cruise, the issue of rudder size really doesn't have an impact from torque and power settings, especially when talking reduced nominal cruise settings.

Reduced nominal cruise are strictly to "save the engine."