I had a hard time replacing the two speed square W.S. motor in my '66 El Camino last year. The catalog replacement unit (from A-1 Cardone) was square, but it did not mount to the existing holes, the base was offset if I remember right (I had similar trouble with the heater fan too), and the arm might have been different length too. Rather than take it apart and try to "make it fit", I took it back and asked to try a '67 two speed square unit - a perfect replacement! I also have a 65 442, it has the round unit, and the mounting holes (probably arms too) were very different from both of the square units when I compared them.
Jim