When he retired, Franco Harris was #2.
Jim Brown had more.
Franco had a great offensive line, and Rocky Bleier was not afraid to aggressively block for him as well. Franco was known to dot a lot of shuffling left and right waiting for just the right hole to open up.
I had an uncle, long since passed, who used to say if Franco had as many yards going forward as he did sideways, nobody would ever be close to him in career yardage :)