Three wire alternator on a two wire system?

I found this M&H harness at Year One (https://www.yearone.com/Product/1966-74-A-B-E-Body/HU228AM), but it states that if I'm using an aftermarket electronic ignition system, I'd be better off starting with what I already have, so that doesn't really help me. I'd stick with the factory electronic ignition if everything I've read didn't say that there are much better options.

I have the three wire alternator and I liked the simplicity of TrailBeast's set up, so I just need to figure out the simplest and most effective way of making it all work together with the factory harness.