The below link tells the story on 70-81 Wheels. I wont go on and on about all the misc details concerning rings,caps &lug nuts, cause I dont think that was your intent. The small oblong 5/8"X1" Valve Stem hole stopped around May-June and the late 70-73 Z-28 got the 13/16"X 1" hole wheels
http://www.nastyz28.com/faq/wheel/z28wheel.html