[Gaz64]

The secondary is connected to the coil negative, as the points open, the magnetic field collapses inducing 400 volts into the secondary x turns ratio gives us 40000 volts.

In an MSD system, a capacitor inside the module is "dumped" into the primary, the secondary is grounded on the common negative terminal of the coil.
The spark loop is coil output tower to the spark plug, cylinder head, engine ground to chassis, to coil negative.

That's why the coil negative is the high voltage terminal, 400v on HEI.