Dan, Is that a V6 or inline 6? If a V6, it will have ADI ignition. You should disconnect the two black/yellow wires at the switchboxes and see if spark returns. If so, your ignition key is likely malfunctioning.
Since Merc did not make an inline 6 150HP motor in 1978, it is either the wrong year or wrong model. You might find out model year by serial number. It likely has a battery powered distributor CDI ignition system.
