Because the "one wire" is using the battery feed as the sensing voltage - the bigger the battery line the better (less voltage drop). Use a sensitive (digital) meter and check the resistance from the alternator connection to the battery (+) post. Mine had too much. So - on the green bird I used a piece of #6 welding cable with soldered copper lugs. Now full charging voltage is available.
Make sure there is no resistance between the alternator case and the other "grounds" - the block, the frame, the battery (-) post. The highest voltage on the vehicle exists between the alternator rotor poles and the case of the alternator - and you would like to be using that voltage. So make all of the grounds as "low" as the case.