It really depends on what tuning your using, and your best bet would be to talk to your custom tuner of choice to see what they recommend. In general, the most common recommendations are usually around the .045" range for a tuned Mustang. If your Mustang isn't tuned, then you can stick to the OEM ranges of .049-.053". That being said, if you are going to tune your Mustang in the future, you probably want to gap tighter now, rather than having to re-gap later.