There are a few differences between the 98 and 99 one being the style of valve cover, 98 has a perimeter style and 99-02 has center bolt so possibly how it hooks up could be different. 97(vette) -98 ls1's were kind of gen 1, 99-01 gen 2 and 02's gen 3 as they all have some differences but for the most part are interchangeable. If two are listed then 98 would be what I ordered.