Most aftermarket parts are designed against the factory part they replace, not against each other. A downpipe is drawn to meet the original system, and a cat-back is drawn to replace the original system from the same point. Both can be exactly right and still not meet, because neither was drawn to the other.
This is not a defect in either design. It is the normal consequence of two designers working from the same reference and then the owner replacing the reference at both ends. Once both ends have changed, the interface between them has no owner.
The same nameplate can carry different pipe routing, different hanger positions and different flange orientation across its production life. A part listed for the model may have been drawn against an early car, and yours may be a later one with a changed layout under the floor.
That is why the model year and the market matter as much as the model name. Two cars sold under one badge in different countries can differ at exactly the point you are trying to join, particularly around the subframe, the fuel tank and any emissions hardware added for one market only.
Neither drawing is authoritative for your car. A drawing describes a part; the car describes the space. Where they disagree, the car wins, and the only useful measurement is the one taken from the interface as it currently sits on your vehicle.
So measure in place, with the car at ride height and the suspension loaded. A car on stands sits at a different height, the pipe hangs at a different angle, and a dimension taken there will be wrong by the amount the suspension has drooped.
Build from the end that cannot move. Usually that is the turbo outlet or the manifold, because it is fixed to the engine and its position is not negotiable. Work backwards from there, fitting and checking each joint as you go, and let the last part of the run be the one that gets adjusted.
Doing it the other way, starting at the tail and working forward, means every accumulated error arrives at the joint that has the least tolerance. The engine end cannot absorb it, so the error ends up in a gasket face or a weld, which is the worst place to carry it.
Before cutting or welding, write down the numbers that describe the junction: the distance between the two flanges with the parts hanging free, the angle of each face, and the offset between their centres. Those three figures tell you whether a straight adapter will work or whether the joint needs a transition piece.
Photograph the parts laid out by material as well, with a rule in frame. When a second part arrives from another source, that record is the only way to tell whether the new part or your measurement is the one that changed.
With two parts and one car, there is no majority. The tie-breaker is which part agrees with the fixed end, and which one has to be modified to reach it. The part that reaches without distortion is the one to keep, and the other is the one to change, regardless of which cost more.
Where both are far from the car, the answer is a short transition section made to the measured interface rather than a modification to either part. That keeps both parts saleable and puts the compromise in one piece you can replace on its own.
