SSI distance between header outlets
 

Does anyone know (or can easily measure) the distance between SSI header outlets?

I have a muffler whose inlets are 579 mm (22.8") apart and I'm wondering if that will work well with SSI headers.

I don’t know the official number but I had to spread my SSI’s quite a bit to get them to fit my new M&K muffler. I first thought the difference was too large to work but by levering things here and there I got it to fit. Not ideal but nowadays it seems one must manipulate a large percentage of aftermarket and even Porsche stamped parts to make them work.

If no one else chimes in I will measure mine tomorrow for you.

OE early exchanger outlet spacing is 580mm or about 22-3/4".
Our mufflers are fixtured to that specification. Original SSI's are spot-on, the new version not so much. It's best to leave everything loose and tighten all up at the same time once lined up enough to get the bolts in.

As Brian pointed out, it's not the mufflers, it's the Dansk built SSIs. When I got that first set years ago I thought I was going crazy. After measuring a bunch of mufflers (with virtually identical results), I realized the SSIs were out of spec. I used a ratchet strap to apply a little tension to the flange ends and the muffler bolted right up. After thousands of heat cycles, they now line up without effort.

The bolt spacing is okay on my Dansk SSIs, but the bolt holes on the flanges aren't colinear. For a long time I just used M8 bolts instead of M10 bolts to get enough slop but I kept chewing up gaskets. I finally got it right, though, by bolting up one side solid, applying heat, and then using the muffler as a lever to twist the collector pipe enough to get it to line up with the other bolt holes (which needed adjusting of their own!).

Exactly this, worked for me many times on a 2.2L, 3.2L, SSIs with M&K, SYCIP, and standard

Awesome, that's the bit of information I was looking for.

Residual stress between headers and muffler makes me slightly nervous since this is ultimately being reacted by the cylinder heads. But I guess the engines keep running just fine?

The heads don't get nearly as hot as the exhaust pipes, especially right after the exhaust port. Don't worry about that. Under full power, the exhaust pipes get red hot.

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.

Ditto this. ^^. I used slightly larger SAE bolts and drilled the flange holes a tiny bit to get a precise fit.