Not being smart but it seems a post like this comes up about once every few months and the poster gets many different answers that can really confuse them. I believe one of the Mopar mags about 6 months or so ago ran an article and said basically any Mopar electronic ign will work fine without using a ballast resister and told everyone to pretty much throw away your ballast if you have electronic ign meaning no points. At least thats the way I heard it.
I know I started racing muscle cars in the early 70's and most still had points then. I also know I saw a few guys burn up the chrome MP ecu by using the wrong or no ballast or the wrong coil and I saw a few coils go bad. In fact MP had a section in their eng book on what ballast to use with the different ecu boxes they had out there in use. And to this day it still seems to confuse many.
Do I understand this right as the original poster here is using a MP ecu and bought a Petronix coil to use with it ?? I am guessing you are using a MP dist ? Honestly my best advice would be to either go on the Pertronix website or call their tech line and talk to them and tell them just what you have and see what they recomend since it is their coil and they designed it. No matter how much we tell you we are not the engineers who made it and if we tell you wrong you will be the one on the side of the road with a burnt up coil or ecu. Like I said I dont mean to sound like a smart a$$ as I can also tell you what I recomend but myself I have not used that coil ever on my cars.
Bottom line is the current will go thru the coil and thru the ecu to ground so the wrong current could damage either one. Your ecu will turn the primary circuit on and off but what ever current flows in the system (2 amps or 6 amps) will flow thru the coil and the ecu and can burn up either if its not right. If you use the wrong or no ballast you may burn the coil or ecu up. We can tell you to run no ballast or a .25 ballast or a 1.5 ballast. Many will work but you want the setup that will keep working and not damage any of the parts.
My advise of course is carry spares to be safe and call Petronix and get their advise. Ron