So, who would be the last sprinters one could be sure were "drug free"? Hines, Greene, Tommie Smith, Bob Hayes, Henry Carr; and on the women's side, Wyomia Tyus, Wilma Rudolph? - though it is widely said Evelyn Ashford was always "clean"
It's well known that members of the '64 and '68 Olympic teams used steroids. Why not, they were legal. I have no idea if any of the people you name used them, but they certainly could have, leading to the wonderful gray area of clean vs "clean"