Installing MSD6AL
Well then the easiest thing to do is simply install a new ECU on the car. Halifaxhops collects old stock, tests if them if they are out of package and will sell you a good one. Also so far no bad reports on the new ones labeled HiRev 7500.
Weak points on the typical '74 electrical system are headlight circuit, seat belt interlock, and column connector.
I also bypassed the ammeter.
I assume this is not a Duster/dart sport with rear window defroster. Hopefully you did a good job on the connections. Some early 70s cars had leaks that did effect the wiring in that area. Not sure if that extended into '74. You've left yourself blind. A voltmeter tied into the switched power at the fusebox (accessory circuit) will provide some indication of the power supply operation.
I have a 4 prong ballast so just trying to figure out how it would be vs the 2 in the diagram
I plan on keeping the original stuff all there in case something goes wrong with it and I’m stranded.
OK. Then probably best to follow the MSD instructions for leaving the ECU connector in place, or at least making it easy to reinstall. Let me find that for you.
The reason for the the dual ballast is the original ECUs used a second ballast to control the current powering them.
The 1/2 ohm resistor did its usuall job of controlling current to the coil.
The 5 ohm resistor was for a second power connection to the ECU. Eventually Chrysler was able to change the internals so the ECU had a regulator inside. After that, the 5 ohm resistor and 5th wire was not needed.
https://www.web.imperialclub.info/Repair/Lit/Master/292/page05.htm
If I do that should I just tie the yellow fan wire into the ignition run?
No idea. We don't know what the yellow wire is. If you show us which terminal it connects to on the relay, then we'll know if it is the trigger or the main power.