Have an intermittent short in the ECU wiring. Tracked it down to where it plugs into the ECU. Engine runs rough in drive, traced it down to the brown wire that goes to the dizzy. This is the original harness for a 74 318 so I have the 4 post ballast resistor. Looked up replacement for harness on Summitt. Can I use this harness as a replacement

I'm not sure what you are talking about.

The Mopar Performance replacement pigtail for the ECU gets all the wiring required to plug the 5 pins module setup, BUT the brown wire for the ballast which is actually part of the engine harness itself.

The only wire running ORIGINALLY between Ballast and ECU is the green with red trace wire which runs straight to secondary resistor on ballast, not used on 4 pins ECU, and even less when matched with single ballast. When using double ballast setup, this wire gets it source throught the ballast with a jumper wire between ballast prongs on blue wire coming from ign switch.

The blue with yellow traces which is the keyed source for the ECU actually splices into the main splice down the harness tape just right before the regulator which spreads the RUN circuit ( blue wire ) to reg, ballast, alt field, and of course, ECU ( choke also on the run to alt field since 73 and interlock reset button since 74 ), just that when adding electronic ignition system on an originally points car, this wire uses to be spliced into the blue wire coming from ign switch at ballast or also regulator just for easier.

Brown wire runs from ign switch to ballast just to bypass it, then to +lead of coil. Doesn't go anywhere else.