96-98 had the same catback, same exhaust. difference was the hangers on the 98's were different for the catback. same 2.25" tubing up until past the axle where the tips are 2.5". 94/95 GT's had 2.25 throughout (even the tips).
and the 96/97's did have the option for manuals to upgrade to a 3.27 ratio from the 2.73 (auto's had 3.08, thats it). i know this because mine has that upgrade, as well as 2.5" tips. also, on the 98's the section under the rear bumper was painted to match the color, whereas 94-98's were flat black. wheels were also shiner on 98's.
and the power difference came from the tune. the heads were all the same for 96-98.