How to choose a ballast

Let me clear this up. On an MSD, you normally hook the "large" black and red power wires direct to a hot battery source. These are what supplies the "heavy" power. The "small red" is an "enabling" wire or "triggering" wire like a relay wire. The idea is that it really does not matter if there's a ballast in the small red wire or not as there is no real amount of current flow in there.

In some MSD setups, you can wire them up in such a way as to quickly be able to disconnect the MSD box in case of failure and resort to the original ignition. This may be why they show it that way.