Impedance is the same as Ohms in a DC circuit but impedance is used in AC circuits as a measurement of resitance to loading and unloading those circuits, according to my old Webster dictionary any ways I was told by one of MSD engineers it was the measuremnt they used in measuring the build up of the charging and discharging thier coils and inside the circuits of their multi spark boxes. MHO is the term used to measure conductance, I'm not sure what sign is used to measure impedance
As far as his recomendations we only talked about the Flame Thrower distributor, not a particular version of it. I have some of the special coils Accel and Mallory coils used only for racing that have a different ratio of winding in them compared to a stock type coil, he said DO NOT use them