The external regulator on my 68 has been long gone, but as I recall, the large red wire goes to the horn relay which is mounted right below the reg. The big red wire goes to the top lug and is held by a scew.
Here is a link to a diagram that should help. Instructions are for converting from an alternator with an external regulator to one with an internal regulator.
Blue, white, orange, brown is the order of the wires on the flat 4 wire regulator connector from top to bottom. When converting to an internal reg the blue and brown wires get tied together and the center white and orange get disconnected and taped.