If it lights off when you release the key that means your problem is in the 12v switched ignition feed to the MSD box. You have 2 wires on the the ignition switch circuit going thru the firewall that provide 12v to the small red wire (switched ignition input) to the MSD box. When you are cranking the brown wire has 12v, when you release the key to run the blue wire has 12v. Start at the MSD switch ignition wire and work your way back to the ignition switch. Don't forget to check the firewall connector.