Well i have a 98 V6 stanger and i was looking for headers and duals for it and anyway so far everything ive found has said "will not fit 98 v6 mustang".
So whats different about the 98 that would make the headers not fit???
Sometimes it is just a fotment issue where the headers will rub against something under the car, or you might end up rerouting some lines in the engine bay. In either case, nothing a cutoff saw can't fix, or adjusting lines.
There isn't a lot of discussion on this topic , and seem that not many even have a clue as to why manufacturer's say this, but I did read where these were the two main reasons.
Another reason they claim LT's don't fit 98's is that there is a small difference in the placement of the EGR tube for the 98's, and no one wants to make a one-year specific LT. YOu might be able to get away with using LT's for a 97, but I'm not certain.