I just did the Dyna install and I opted for the most expensive but easiest/full-proof method of fitting it.

First I got a Dyna, part number 65747-94.

Then I bought the neck-down/angle adapter from Ryca (http://shop.rycamotors.com/exhaust_adapter.html).  It's 50 bucks which is a lot for just a pipe with a slight bend in it but it adapts the neck size perfectly so you don't need to mess with the tin can gasket.  You also don't need to hack-saw your Dyna either.  It really brings the "slip on" concept back to the Dyna slip on.  And for me, that cost was completely worth it for situations like you describe when it worked once and now doesn't.

The Ryca comes with a chrome clamp but you need two clamps to mount it on the bike.  So I ordered an identical chrome clamp (same part number and manufacturer as the one Ryca ships you) so they'd match.

That clamp is just 8 bucks here:
http://www.dimecitycycles.com/vintage-cafe-racer-caferacer-bobber-brat-choppe...

I also replaced the bolts that those clamps have with some grade 5 stainless steel.  The bolts they come with just bend like butter once you start tightening the clamps.

With those three parts (clamp, adapter, Dyna), the install was a complete walk in the park and works like a champ.  You just remove the header pipe, remove the stock exhaust, line up the adapter BEFORE you pound it into the Dyna with a rubber mallet (it's a tight fit), and then mount the Dyna using a single mounting hole on the frame only because the exhaust adapter moves the exhaust backwards so both holes don't line up with the bike mounts but one hole is totally enough.

The last thing I bought was a 4" heat shield cover from Samson (http://www.samsonusa.com/H-109.html).  Again, it's 30 bucks for a bent piece of chrome but it covers up the muffler mounting beautifully and is short enough that the 10 degree angle adapter isn't a problem like it would be if you had a 7 inch heatshield.

I think I've heard Serowbot say rattling is a sign of a rotted muffler.

As for the original heat shield, I couldn't use mine because it was welded to the original muffler.

Here's a picture.  I pushed all the pipes in as far as possible and there is a small air leak at both connections.  I used some grade 8 bolts but they eventually started bending too and I couldn't tighten them anymore.  I'm out of ideas.

I guess I can live with a little air leak and small backfire?

justin or sero or someone always chimes in on such cases with 'exhaust soot will soon plug those small air leaks'.  lol.