Any combo of header and exhaust that is designed to bolt up to the stock cat is going to maintain the same 2.25" inlet and outlet diameters. The stock cat doesn't really become much of a flow restriction unless you go to 2.5" piping or it gets really old. Since you have done neither you might as well just keep the stock cat.

As a side note, Carsound only sells universal-fit cats. You need to have flanges and oxygen sensor bungs welded on after you purchase one. Some places sell them pre-made with all of the welding necessary for a particular application. Still, on a '98 with that kind of mileage and 2.25" exhaust piping the stock cat is not a problem.