This a 75% scale P-51 designed by Jim Stewart of Vero Beach, Florida.  Unlike most kit built airplanes, this is true to the original except for the scale and power plant.  The V-12 cylinder Merlin engine was replaced by a 454 GM truck engine with a gear reduction to the prop.  Due to the increased speed of the V-8, firing frequency is about the same as the V-12.  But nothing can duplicate the sound of a Merlin.

The original gears are shown on the left and our gears for the V-8 are on the right.
Performance of the Stewart-51 was outstanding for the size engine but a little short of the actual P-51.