70 Duster keeps shutting off

Get a voltmeter and hook it up to the power feed to the MSD box when it acts up; put the ignition in RUN and see if you get a voltage to the MSD power feed that is no more than a few tenths of a volt lower than battery voltage. If it is good, then suspect the MSD box but I will not be surprised if you find that voltage pretty low. The fact that the oil light goes off and then comes back when things are ready to start indicates a problem in the ignition switch area or its connector.... which it sounds like you have not yet addressed.