changing from points to electronic

A ballast resistor is a superior design to either a fixed power resistor (or resistor wire) or to a coil thats internal resistance is sufficient to allow it to operate a full battery voltage.

Besides the obvious of allowing it to be bypassed during starting for a hotter spark the ballast changes its resistance over the operating rpm range to compensate for the reduced output of an inductive ignition as the rpm goes up.

An inductive ignition works based on a collapsing magnetic field in the primaries of the coil by the points opening or an electronic module switching the current flow off. To get max potential from the coil the current needs to flow for a given amount of time to saturate the secondary windings. But as the rpm goes up the time for the saturation to occur goes down and so does the potential output. This can be compensated for by increasing the voltage in the primaries. If you increase the voltage so you get reliable ignition at high rpms there is to much current flowing at that voltage at low rpms that can result in damage to the coil from over heating.

A ballast by definition changes its resistance with temperature, the hotter it gets the higher the resistance. So the ballast resistor at low rpms has current flowing for longer periods of time between firings so the resistance goes up thus limiting the voltage and current to a safe level. At higher rpms the current is flowing for shorter periods of time so it does not get as hot and it allows more voltage to the coil.

The GM HEI system uses a completely different coil design that is less susceptible to over heating so it can be designed to provide adequate output at high rpms without the danger of over heating at low rpms. Use a GM module with a Mopar coil without a ballast and you are over stressing the coil and reducing its reliability.

The more modern electronic ignitions normally use an E-core coil that is more like the original GM HEI coil and the electronic modules are more sophisticated and can control the current flow by adjusting the dwell (the time the current is flowing) over the rpm range.