1975 Dart Manual steering conversion

For me, the easiest way to tell is the diameter of the threads and/or size of hex nut below the pitman arm. In the mechanical standards a hex nut is 1 and a half times its bore. So the earlier smaller is a 3/4 threaded shaft with a 1 and one eighth hex nut.
The later and larger is 7/8 threaded shaft with 1 and five sixteenths hex nut.