Did you get the correct harness? The '65 GT harness I know for sure is different.
If you have a multimeter, using the resistance scale, make sure that the FLD lead on the alternator goes to the "F" terminal on the regulator. The STA lead on the alt goes to the "S" terminal on the regulator. And the BATT lead should go to the (+) side of the solenoid and to the "A+" terminal on the regulator.
If you have a multimeter, using the resistance scale, make sure that the FLD lead on the alternator goes to the "F" terminal on the regulator. The STA lead on the alt goes to the "S" terminal on the regulator. And the BATT lead should go to the (+) side of the solenoid and to the "A+" terminal on the regulator.