Yours might have been 12 3/4's when new, the springs can sag after so many years. Any shock in the 12 1/2 to 12 3/4 inch length is probably stock. The next step down is about 11 1/2 inches, some like 10 1/2. As you shorten the lenght watch to see if any thing rubs, tire on fender, chain gaurd on frame, stuff like that.