Before 1971 there were no GNs at all. The last race of the night was usually called the Helter Skelter, and the final winner did not start with a handicap.
When GNs were introduced, the handicap was officially "half a lap", and I seem to recall that the final winner lined up halfway between the front of the whites and the back of the superstars. At some point (anybody know when?) this was changed to a full lap handicap, with the final winner starting in directly in front of the whites.
I remember my dad telling me about the first time Stu Smith won the final after the full lap handicap came in. He loaded up and went home before the GN, after saying on the mic that he didn't think that winning from the full lap handicap could be done.