[RarlyL8]

Misalignment of the header outlets and the muffler inlets causes a lot of issues.
First off is the difficulty of installation. Everything has to be left loose and tightened up after all of the bolts are in place. If not you end up cranking on one particular problematic area which puts all of the strain on that flange. That is the second issue. Best to distribute this stress throughout the headers and muffler if possible.
Third issue is easy to overlook and that is the misalignment of the ports. If the headers and muffler don't line up forcing it will push the mounted bolt holes to their extreme which pulls the flow path hole off-center. To combat this we use very close tolerance M10 bolt holes on both our header outlets and muffler inlets. The bolts fit very tight which keeps the flow path on-center. Some aftermarket flanges have oblong bolt holes which are great for installation but really bad for assuring proper alignment of the header outlet to the muffler inlet.