I had been having intermittent starting problems over the last couple of years in my '66 and was getting worse and worse. I checked all ground and connection point voltage until it we tracked down a voltage drop in the circuit between the ignition switch and starter solenoid.
I ended up wiring a secondary "Start" button and relay directly to the starter as described in a few other threads. Works like a charm. You can see the new button on the dash where the lighter used to be.
Regards,