Why do you want headers for your 98 stang? The 98 headers actually flow pretty well even though they are butt ugly. New headers won't put out a big difference unless you are pushing high horespower numbers. JBA makes headers though if you have to have them... and you can get a set of long tube or shorty headers from MAC for a 97 and have them modified to work with our year's setup.
Well, if your going to get headers and plan on moding the car preaty well I would just get long tubes. you can get them from ebay, rpm-mustangs.com, speedconcepts.net, rpmoutlet.com just to say a few. Any speed shop should know what they are doing and can get them to work on your car. but, headers are no good unless you plan on moding your car well, like heads, intakes, cam stuff like that.
The reason you need to get headers from MAC modified for our car is due to the egr tube setup in our year. It is just a little different and everywhere you go you will see a disclaimer telling you that if you have a 98' it will require modifcation of where the EGR tube enters the header. All you would have to do is take it to an exhuast shop and they would know what they have to do. However, with JBA I have not seen the same disclaimer for 98's so they might not need modification. Again, headers are going to do next to nothing for your car unless you are pushing high horsepower applications. Spend the 300 dollars or so you would buy headers with on getting a set of gears and t-lok, or nitrous, or CAI, or cam... all of which are proven for power gains.