Dougs

This is really true of all aftermarket parts. Once you start mixing and matching aftermarket stuff you really wander into unknown territory. Each part may have been produced to work with the original stuff, but they sure aren't all intended to work with each other. Some will and some won't. And even little stuff makes a difference, I know for a fact that my engine doesn't sit in the same place with the replacement poly inserts in my spool mounts as it did when the original rubber was in there.

The headers always end up being a headache because they depend on everything. The engine mount locations and type of mounts, K-frame alignment, oil pan, steering box, torsion bars, steering column mounts, shift linkage etc, all come into play. All of those parts have independent tolerances and adjustments, and that's just from the factory. Replace the engine mounts, add an aftermarket oil pan, increase the size of the torsion bars, install aftermarket heads, steering box etc and things get entertaining. Not to mention just the factory assembly tolerances on these cars, or the tolerances on the production of the headers themselves. Not like they all leave the factory identical to each other, even if they're all built on the same jig by the same person (and they aren't). Sure, they're close, but when you start routing things through with an 1/8" of clearance you're going to run into problems, because the factory never spec'd any of the body parts that tight. Even the suspension points had that much tolerance or more depending on the part.