Hi Torque Starter was added with a relay:

 

Original wiring of the Starter.  The small red wire is from the alternator harness.  You can clearly see that the solenoid ground goes to the starter motor by design, but DO NOT USE that as a ground for a starter relay, the pulses with cycle the negative on and off.

 

Original 72' Setup without the Ford-stye relay in view..

 

 

Original 74' setup with the Ford-style relay: