Well, the spark plug catalogs don't distinguish between models because they aren't always right (customer swears it's a GT, or maybe an LX, or was there a GX?), and the VIN engine code is much safer. The spark plug catalogs I looked at list two different plugs for 93 5.0's. That's it. Nowhere is GT, LX, or Cobra even mentioned. It's kinda funny, though, because that changes for 1994. There they have "Cobra Mustang" in parenthesis after one listing, and plain "Mustang" after the other. They have the same engine codes listed as they do for 1993, though.
Screw it, I'll just show you what I mean. This should help you understand what the kid behind the counter is dealing with:
Notice that the 3 5.0 listings for 1993 have no Mustang designation at all, but the same two engine codes are listed in 1994 as Mustang and Cobra Mustang. The guy really had no choice but to ask for your VIN.
Take care,
-Chris