Won't start one day and fires up first crank next day

He may be "mis using" the term. HEI was originally GM, period. Since then it has gotten "spread out" into aftermarket and several types of ignition, even though "quite high output" are not "copyrighted" GM terminology. HEI to me means "using a GM module" or a replacement module designed to bolt in replace a GM module

BY THE WAY many of us has DONE that--replaced the Mopar ECU and eliminated the ballast, with various kinds of GM module

I usually don't suggest such things during troubleshooting, because you can add problems and complicate the matter