Four 16 team super-conferences of two divisions each = 64 teams. Win your division and play for conference championship. Four conference champions play for national title.
OK, what about #s 65 through 68? Every year the worst team in each conference is relegated out of the conference, and the previous #65-68 join their new conference. You could base relegation on a rolling three or four year average if you like.. Try to keep geography relevant to conference assignment if possible, but no guarantees.
If you’re #69, tough!