Headlight harness comes out of the firewall with a Packard 56 type plug, if original it could be pretty corroded in there and difficult to pull apart. It's not the only thing in that plug as shown ion this diagram. There is a 5 wire plug and a 3 wire plug coming out of the same grommet on the firewall up by the master brake cylinder.
http://67-72chevytrucks.com/vboard/s...d.php?t=629550
And the firewall holes shown here:
http://67-72chevytrucks.com/vboard/s...00&postcount=5