i have one of those acton digital meters and in order for it to work on my 72 340 duster with msd ignition i have to hook the positive lead to the tach output on the 6al box.
Just a FYI, With the MSD, be carefull, connecting ANY test equipment to the coil. The MSD supplies several hundred volts to the coil (not 12 volts), and can burn out some test equipment.
