Hy stans6400, welcome, in Canada where I live C10s have six lug wheels, C20s have eight lug wheels. Your trucks have been around for almost sixty years, who knows how many times the wheels have been changed. As far as I am aware trucks never left the factory with wheels of more than one type. You will have to do some research to find out what the wheel and tire options were when your trucks were new. You can decide which wheel and tire combo you want to use after you review all your options. Again welcome.