Really, '01-02 are the “preferred” years that tend to dyno the highest, and have the LS6 style intake manifold (no EGR). '98 is the year generally to avoid because of the different PCM and thinner sleeves. That being said it really isn’t anything to worry about, or enough to avoid '98s just expect a better price. My buddy has a '98 Z28 with about 180k miles now, over 100 AutoXs, a few dozen track days, and a few trips to the strip. It was his DD for a long time too (as the miles show).
-TJ