JHJ was a beast, at one time ranked 4th on the NFL's all-time rushing list, behind Jim Brown, Jim Taylor, and his 49er teammate Joe Perry, with whom he shared carries in the million-dollar backfield that included Hugh McElhenny too. JHJ is a Hall-of-Famer. In one of those great Saturday night absolute brawls they used to play in the old Cleveland Mausoleum, Oct 10, 1964, 35-year-old JHJ outgained Jim Brown 200 yards to 59 yards. I remember seeing those games on TV. JHJ was a Steeler from 1960 to 65. Maybe a bit old for some on the board. He would be in the top few on my list.
I would have included both John Henry johnson and Dick Hoak in my list but the OP's premise was that no RB who played before the 70s should be included so I left them out.