they fit just fine when i put them on. i didnt have any problems with anything binding up or hitting. where the collector pipe hooks up to the down pipe to is a little short so make sure you tighten the 2 bolts down a lot. i didnt notice any milage gains from the header because my cat was all backed up, there is an exhaust leak where the header bolts to the head due to a broken stud and i had to drill and tap. i did notice a little performance from the header but as for how much id added, i couldnt tell ya for sure. you could hear the exhaust note change to more deeper rumble.
as far as my intake/exhaust set up, i have a 92 XJ 4.0LHO with a home made k&n cold air intake, poweraid throttle body spacer, gibson header, 2.5 downpipe, high flow cat, 50 series muffler, and 2.5 tail pipe. did that cover enough?