Also worth noting - HEI isn't the only ignition with variable dwell. The Bosch systems used on most early '80s European cars, Ford's TFI ignition module (the one that replaced the Duraspark), and pretty much everything fuel injected where the EFI computer had timing control integrated all used variable dwell. Variable dwell also removes much of the need to match the ignition box to the RPM range (at least until you get to the point you need a CDI) since the box can change its dwell with RPM. You won't see serious overheating issues with a properly working street box on the street, but try using a gold ignition box and its ballast in stop and go traffic and you'll run into heat issues. A good quality variable dwell ignition can pull off gold box behavior at high RPM and street box behavior at idle.
That said, it's an incremental improvement, and may not be a difference for your build. I would not bother swapping the distributor if you want HEI unless your distributor is damaged beyond repair - wiring a good quality module to your distributor and possibly swapping the coil are all it takes. And if you already have the advance curve dialed in, swapping the distributor means you now need to copy your advance curve into your new distributor to get it to run as well as the old one.
And there's a reason I put "have the advance curve dialed in" in bold. The right advance curve is a lot more important than the HEI vs Mopar ignition debate.
