I can't see the pics but there is no such thing as a center exhaust for the 94-98 Saleen rear bumper. If there was I would have it for sure. That's not to say that someone could make one for you.
There is an aftermarket company who makes the Saleen style body kit for the 94-98 body style cars that has the 99-04 look. I'm not sure if they offer a center exhaust piece. That company advertises on e-bay often and the model car is white. Never seen the kit in person.
If you are talented with a dremel tool you may be able to cut out a center section and you can purchase the Chicane center exhaust from Joe at Chicane. IMHO Joe's center exhaust looks better than the S281E center exhaust that comes on the factory cars.