So I got the exhaust I purchased on ebay in, made a bracket, had an adapter made, and installed new t-clamps.  But I can't get the muffler-to-adapter connection to seal well at all.  I've torqued the clamp as tight as I dare as I don't want it to pop!

The adapter pipe was flared to fit the muffler, and I don't think it is strong enough to hold the clamp pressure.  I was thinking about having it re-flared and a second piece of pipe flared in the end also to "doublewall" that section.

I'm all for redesigning if anyone can think of anything better.

I put high temp Permatex on all my cycle's exhaust. Let it dry 24 hours before running. I have 2 years on my Savage's Supertrapp install without leaks.

Do the outside pipes have a lengthwise slot in them. You could cut one with a hacksaw if you have none. You could possible weld the rear connection but you'd need to put a little silver high temp paint on it to keep it from rusting.

Use good clamps. I used to drive over to Midas and buy clamps when I did auto exhaust work. Maybe they have small ones to suit what you're doing. Check out the Harley clamps too.

What max said about the high temp RTV.  I've been trying to kill an header to muffler exhaust leak for about 9 months (well, not very hard, anyway!).  Just blows through it after a while, and if you don't get it right the first time, forget it!  The stuff just seems to rot, too, over time.

Cut a 2 inch wide strip off a soda can and wrap it around your header pipe and adaptor like a shim, if one is not enough use 2. That's what I did, works great and NO leaks.
Just my 2 cents

Ike