From what I know Ford changed the exhaust header configuration a little and that's where the extra 10 hp comes from.
As far as the taillights, as far as I know they are the same 96-98. I personally haven't seen a difference between 96-97 and 98.
Stefan how do you like the B&M Ripper? I'm trying to decide between it and the Steeda.
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1996 Mustang GTS