There is a resistive wire from the firewall bulkhead connector to the Positive terminal on the coil.
This wire, plus IGN position of the ign switch provides run voltage after the engines starts. Starting coil voltage comes from the starter solenoid, yellow wire.
So, if an MSD is there now, this resistive wire was reomoved from the + post to a wire for the MSD unit. Some times there may be a ballast resistor there.
I would go back over your electrical hook-up and check surrounding areas for pulled wires, etc.
It could be the ignition switch itself, think of the age of the car, now multiply that by how many times it has been started, Quite a few times, huh?
Good luck, let us know how its going.....