To begin the test, you could bring in the 400 hp engine used in the actual vehicle and a water brake or other device to put a load on the system, or you could consider other ways to develop the forces encountered by the drive system. If we know the speed of the engine is 4000 rpm when the 400 hp is developed, then we can solve for the system torque as follows:
Torque (ft-lbs) = hp * 5252 / rpm. So, 400 * 5252 / 4000 = 525.2 ft-lbs or 6302.4 in-lbs.
Now we know the speed of the engine and the torque or amount of force in our system. We can use an electric motor to drive our system and use a rotating actuator such as a SL-3V-750 to provide the force or torque in the drive system. The electric motor only needs enough hp to overcome the frictional losses of the system. See an example below of a power loop or four square test stand with two automotive differentials.
In this example the motor is driving the first differential and the shaft of the actuator. The actuator body is connected to a second differential through a ring gear and both differentials are tied together through gear boxes. We can now use a low hp electric motor to drive the system. The actuator while spinning at the system rotational speed will provide the force component of test. This force can be varied, by varying the differential pressure across the stationary ports of the hydraulic union allowing the actuator to spin.