68 Barracuda Formula S 340 car Charging

If there is no connection to one field terminal, All you have to do is run a wire from that field connector to ground and it will work fine. It doesn’t matter which one is grounded. The 69 back ones are supposed to have that done.

You can ground it to a bolt on the alternator.

You may also be able to ground it by removing an insulator from the brush holder so the mounting screw grounds it. If you google it you will probably find some images.