After trying the tin-can two times and still leaking, I tried aluminum foil from a dollar store pie pan. That easily wrapped twice around the pipe and stayed in place while I placed the muffler over it. The muffler clamped down on it well (Cobra 1.75 t-clamp).
Still no leaks after about 300 miles.

Use an adapter between header and muffler as per Dave's measurements (Dyna muffler adapter in tech section) with t-clamps and no leaks whatsoever. No need for any shims.  Still allows for using stock mount one bolt with dyna muff.