coronets 1975 exported to Kuwait , six cyl. - why installed diff. of v8 cruise on these series ?

My dady bought a1975 Dodge Coronet six cyl. manual transmission  brand new in Iran -Tehran . the car was made to be exported to Kuwait but it ended in Iran . No power in first , second or third gear , no matter the road is up or down hill . I was 16 years old in that time . No mechanic could guess whats wrong with the car . After almost 20 years the car was still in good conditions , I was suffering why the car is so so so lazy that can hardly move his body ? I studied this very carefully and i shot at the target , i just changed the diff. with a lower , much lower ration one , now the car is able to go vertically on a wall . I am hundred percent sure that no one before me touched the diff. having in mind that the car was absolutely brand new . Now the question sounds like this , why did Dodge installed a v8 diff. cruise on a six cyl. engine ?