For all 67s/68s, the backup switch (manual transmissions) and neutral safety switch (automatic transmissions) connected to a 4-prong pigtail (2 male and 2 female ends) that came out of the upper part of the firewall. There was no intermediate harness to connect the switch harness to the pigtail. If yours does not reach, then your routing may be wrong or the switch is wrong. The switch harness sort of followed the top of the transmission up the bellhousing where a metal clip attached to a top bellhousing bolt held the harness in place.
Regards,
Pete Morgan