68 Barracuda Formula S 340 car Charging

Hey there and thanks.
The back of the Alts I’m getting has a post with a nut and 2 field tabs. I’m not quite sure I understand your question. If there are two tabs one of them gets grounded and the other one has a plug and which runs back to the firewall and towards the voltage regulator?

If I’m understanding your directions correctly I should run a ground wire from one of the tabs to sheet-metal somewhere?

Yep, you can actually just ground one of the tabs to the alternator case easily. As Mattax said, there is a nice picture in post 3 of the modifications. The last picture.