I'm looking to replace the exhaust manifold on my 1990 Cherokee Sport 4.0 I6. I want to go with a steel header that I found at Morris 4x4. Has anyone had any experience with this product? The ad says it will fit 91 to 99 models, but I was under the impression that the 90 and 91 model engines were exactly the same. Besides, they sell another header that fits 87 to 93. How can one header fit a 91 and not a 90, but another header fits 87 to 93? Also, is there a noticeable difference between a header and a manifold?