I have a similar setup aftermarket starter with a built in solenoid on my 65. All I did was take the cable off the starter side of the solenoid and move it to the battery side. I then attached a new 14 AWG wire from the starter side of the original solenoid down to the small terminal of the new starter solenoid. When you energize the original solenoid it will send 12 volts down to the new starter solenoid and crank the car. This way you can keep that "original look" of having the original solenoid in place if that is what you want. Also, you dont have to figure what to do with all of the other wires on the original solenoid. Just leave them where they are.
So far as neutral safety switches are concerned, to my knowledge, US made manual transmission cars have never had a "neutral safety switch" later model manual transmission cars do have a "safety switch" wired through the clutch pedal so it has to be depressed for the starter to energize
Thanks Zigmont. Like I said I wasnt really into manual tranmissions but thats what I thought. For a second there I thought there was a ton of bypassing going on. I can remember having to drive a few stick shift cars that jumped in gear

