Okay, I've never been able to get the muffler on the -6 to mate to the headers
without leaks at either the top or bottom. It seems like a stupid design, with only
the 2 bolts and no strap to hold the weight of the muffler like on the 911.

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I'm using the copper covered gaskets. Is there some kind of trick to making it work?

I've always used a thin smear of copper exhaust RTV on mine. It was the only way I could get them to seal up reliably. Doesn't take much, just a thin circular ring around the pipe.

A real /6 doesn't have a muffler hanger like a /4?
That is odd. If possible I'd fab a hanger like a four.

Coming from a VW beetle/bus (then 914) background every single failure I see like this is related to lack of support of the muffler.
A large number of head exhaust stud failures are also related to this issue.

Most likely culprit is one of the flanges is thinner than the other and when it has been cranked down too hard it bent the flange. And/or this also crushes the gasket at the bolt holes causing a leak.
This relates to the muffler hanger issue, because as the joints rattle loose the owner cranks down the flange bolts tighter and tighter bending the flanges.

All of my 6's had/have a hanger!

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Of course there is the hanger on the end of the heat exchanger to the trans,
this is like the 4. However, the 911 has 2 straps that hold the muffler up, which
was skipped in the 914.

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The 4-cyl cars have bigger flanges with 3 bolts on different lines. The 911/914-6
has only 2 bolts on one line. Not nearly as good.
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I think he is referring to the stainless straps that go around the muffler to support it. The 911 has two. I have the same problem with my new headers and oem muffler. I took my headers and muffler to a muffler shop to touch the mounting surfaces on a giant belt sander. Even after I had a problem with one of them mating up and leaking. The solution was to make sure not to over tighten and the leaky one needed two gaskets. I also used new split lock washers. Recheck tightness after first use.

Thanks will try that this spring

So then it gets back to thin flanges and/or crushing the gasket, plus vibration.

As far as the 911 straps I agree it's a lot of weight hanging out there. I personally would mod the existing bracket or fab a whole new one. A bracket that not only bolts to the flange, but backs up the muffler to use the stock 911 straps like in the 911 mount drawing.

The muffler on all aircooleds take a shit kicking from vibration.

On my Type 4 bug conversion A1 header I use no gaskets, expansion of the exhaust seals it up, but it has heavy 3/8" flanges. Even with that I had to make a muffler hanger to support the exhaust. The A1 original mount was nothing more than a strap around a header tube , no support of the weight hanging out at all.Then as it turned out the wimpy bracket A1 welded to the muffler ripped right out of the muffler. So then I had to re-pack the glass and weld a heavy 2 layer backing for the muffler.
A1 makes a fairly nice header exhaust, but it's muffler hanging is piss poor design. The A1 original mount was nothing more than a strap around a header tube , no support of the weight hanging out at all.
So far it's holding up with my mods.

When I go to install my six, I intend to weld some small tabs on the
hanger that attaches to the trans. Then being able to utilize the stainless
911 straps for extra stability/support. I have a stainless silencer on the
4 that is quite heavy. The hanger/bolts have their work cut out for them
keeping it stable. So definitely adding straps.