Erm... basic free-body-diagram theory here. A moment is a moment is a moment (or torque, or couple, as you prefer). It has exactly the same effect no matter where it is applied - the reaction moment arm, for this discussion, is the distance between the front tyres (where the reaction force is reduced) and the rear tyres (where it is increased by an equal amount) - i.e., the wheelbase - which doesn't change.
The only influence that the position of the motor has is on weight distribution and moments of inertia.
If you could build a car with two motor mounts, one in front and one behind, and you could fit a motor in both positions but only fit a pinion to one of them, then it wouldn't matter which one was spinning.
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