I'm actually going through this very issue with the 72 Duster now.
I installed the MSD system (all plug and play) MSD Box...coil and distributor.
I wired everything up temporary to do the break in and everything worked with no issues.
I had to rebuild a steering column and convert from Power to manual so I bypassed everything to break the motor in.
NOW when I'm wiring everything up complete I have the yellow (Start wire to starter relay) wired up with no issues.
The large Red hot for the MSD to a battery hot...
Black to ground...
Msd run hot wire (Red) tied into the Blue (ignition 1)
Here is where I'm wondering why I wouldn't tie the brown wire into the yellow wire and pic up ignition 2 when the switch is in the start position?
Is there a downside to having the brown wire tied into the blue (ignition 1) all of the time?
Why couldn't it also be tied into the yellow where it only receives juice when the ignition is in the start position?
Just an FYI the Solenoid is a Ford unit and is behind the dash along with the MSD box. The only wires I use through the factory pin connectors are the (purple) engine temp wire and the Black Alternator wire. The alternator is a one wire unit so all other wires are terminated at the pin connector.