A GM module is basically a small MSD without multiple spark. You can mount it on the firewall ,fender ect. Lots of wiring diagrams on the web. It will eliminate the ballast resistor. All you need is a coil, a module and a heat sinc. Less than a $100. Spare module just in case. I doing 2 for my cars in case the MSD fails. Don't use that big cap.
An MSD box is a bit of a different critter: It fires the coil by dumping 400 volts from a capacitor into the coil. HEI uses an electronic switch that behaves like points, but with variable dwell.
This has a few differences in practice. One, a tach that works with the stock ignition will work fine with an HEI. With the MSD, you may need an adapter, and should not wire the tach to the coil. Two, a coil optimized for an MSD will barely work with an HEI, although a stock type coil will work OK with the MSD. A stock coil with an HEI might not work as well as using a modern coil, but it will still do at least as well as it did with the stock ignition if not better.
