spark plug gap

What other supporting mods did you do? You just slap on the heads?

GT-40P's take the same plug as the 1993 cobra. Gap is 0.052-0.056" on 86+ Mustangs (Naturally aspirated)

Gap is more or less dictacted by the ignition system and not the head really.

Stock igntion is good for 0.052-0.056". On some older system,s some will find the engine runs better closed up a bit. With aftermarket ignitions, you might be able to go larger N/A.

Depends on each car really. I say start with 0.054" and adjust from there. A lot of guys like to run around 0.045" gap. A lot of text incorrectly state this as the factory gap. The true factory gap on 86+ 5.0's is in fact 0.052-0.056".