Here is another option to do the ground upgrade for the driver board.
This way you have multiple ground connections all connected together.
https://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_1#Optional_Ground_Upgrade_-_Driver_Board
Check the connector pins at A3J2-2.
https://www.pinwiki.com/wiki/index.php?title=Count-Down#Charts