04GTMustang is on track. Run the widest gap that your ignition system can support. You will know that you should decrease the gap if you get a mis-fire/spark blow out at full load. So run the stock gap. If it misses, then decrease the gap.
As for plugs, it won't hurt to run a colder plug. If the plug is too cold, you'll get a miss at idle because the plug will foul.