I will say I have put a few MSD units on my buddies cars and all of the MSD units I put on that were complete MSD units that used the MSD box never needed the ballast. They are the units that both coil primary wires go to the MSD unit.
That said I remember Mopar Muscle saying you can take the ballast out of the Mopar electronic ign units. Of course there are no points to worry about and the ECU is fed 12 volts with the key on but I was always told the Mopar ECU units still needed the ballast so the coil dont overheat. Well I plan to find out as I took my ballast out a few months ago as I use the orange Mopar ECU. I carry spare coils , ECU's and ballast so I hooked the ballast wires together and plan to see if it keeps running all this summer like this since I want to see if what Mopar Muscle said it true. Good luck with your HEI setup. Ron