When I get this sorted out I will start a new thread but for now
Key in accesory signal switch in left signal the flasher cycles and the starter engages when the flasher cycles
Key in accessory? That accessory position is not avail until 65!
A 63-64 Ignition switch only has Off ,Start and Run.
Hmmm.......Starter engages when flasher cycles.....
I believe you said you had a 64 underdash harness,but off the top of my head a 65 underdash harness has a separate flasher connector shaped like a V could you possibly have connected this to the nuetral safety connector which also has a V? These would be a green and pink wires or green and pink with a green stripe.This can possibly connected to the fuel gauge.
Another thing i would check is the T shaped Nuetral Safety Switch connector connected to the back of the SS cluter? Maybe the Neutral safety circuit is getting back feeded from the instrument cluster temp gauge.