Hi,
Definitely a neat find, nice to see its a post car.
The harness you could just find one out of a 65 Skylark or Special as they would be the right lengths for the later 455 not sure if 67 has the same plug or if the pins are the same between years either way I would sit down with a electrical schematic for anything you go to swap in, you could just lengthen the wires on the coil and starter wires as I did the opposite doing my nailhead swap I just shortened the wires from my original 300 harness and had no issues
Good luck with the project again, not sure if you have had a nailhead with a stick before, but you would be pleasantly surprised at the power and smile factor you will have even from a stock engine and depending on your area and how rare nailheads might be in you area, you will also strike lots of interst at the car shows as I know my flaking paint car does (my wife still doesn't understand)