Two Ignition electrical questions discovered during MSD installation

You have a weak connection at your key switch connector, bulkhead connector, or possibly a wire with an internal break. It's best to correct this before going any further and I would advise to use the key circuit to energize a relay and connect a fusible link wired from the battery terminal of the starter solenoid to the relay contact to energize the MSD unit with the relay. Our 50 year old key switch and terminals oftentimes don't play together too well with modern high demand systems.