How to choose a ballast

There are really two criteria. You have to match the ballast to the coil, and if you are using an ECU.........like a Mopar ECU, then the ECU has to "handle" the coil / resistor combo

This was especially true "back when" you had the "old huge" Mallory big rectangular coils. Those things used the "stock" factory ballast along with the Mallory supplied ballast in series with the factory ballast, and THAT could be a lesson in "how to read" destructions

I actually saw a guy explode one of them once on a 55 Chevy. Installed it, left out one of the ballast resistors, and left the key on while trying to get it to run, and promptly exploded the coil........oil and all......all over the engine bay.

In the case of Mopar stock coils, refer to the specs in the back of the service manuals........there are listings for the coils and ballast resistors. "In the old days" before the Chineseos took over, you could actually look up ballast resistors in the parts books and find out "what they were."