Slant 6 pushbutton to 3 speed swap

Run a wire jumper from the G terminal of the starter relay to ground. That simulates the NSS being grounded (activated) all the time.

And consider adding a clutch safety switch long term for safety. The '64 had no clutch safety switch; that was added several years later as a safety feature. Install the clutch safety switch at the clutch pedal arm like in later years; ground one side of the switch and run a wire from the other side to the G terminal of the starter relay.