I have a 68 that was originally a 400 hd 3 speed. I don't know if this is correct but there is a round plug with 2 green wires (dark and lime green) that comes out of the firewall between the fuse panel and the hole where the clutch rod comes out. Another harness joins it and goes in the direction of where I should have a transmission. This harness has a yellow and a pink wire and appears to have spent an unfortunate amount of time on the exhaust. It does look like what you are looking for. I have been searching for one of these transmissions but no luck so far.