I agree with the last few posts, there's no k-member design reason that they shouldn't fit, there's something else going on.
People that bash parts companies are amusing to me. Unfortunately, it's the nature of low-volume manufacturing- not every set of headers, mufflers, etc are going to fit exactly the same. Hell, even plastic parts that come out of the same mold don't always fit the same way. Over the years I've installed headers and mufflers from nearly every manufacturer, sometimes multiple sets on the same car, and even the "best" brands sometimes need some finessing to make fit. not trying to brag in any way, but just sharing experience. My passenger side long tube was a bitch to get in, if I recall.
Even from an OE-level, take the factory Mustang glass roof as an example of build variation. The factory roof as two large gloss black filler panels at the front and back of the glass roof panel. It is fact that these panels are used to account for variations from body to body.
I understand you are upset that you can't buy one header, but also understand why they cannot break up a set as well. Good luck with the install...
