Yep, Dave has hit it. Your plugs are just too long. Let's face it, the only thing that can close up the gaps is something hitting the plugs.

It really makes it tough when you are dealing with something someone else put together, there is no telling what years or parts are in it.

Just take your plugs to the auto store, and start looking at plugs from other year applications, Mustangs, LTD's, etc.
When you find one that is a little shorter you will be in the ball park.

Evidently your plugs aren't way too long, as you would have seen more than closed gaps.

Don