I got a pair of Dyna mufflers taken of a brand new bike. The interior diameters, weren't the same. I used the larger one. I assumed it had something to do with the different lengths of the Dyna's header pipes, maybe something to do with balancing back pressure, or something like that. Just guessing though.

I installed a freeze plug last night...Upon inspection the interior pipe with holes looked to be mostly intact.  So I drove the plug about halfway in, thus forcing gas into the first holes and out the last holes.  Kinda hard to see that far in there so I guessed.  

Erring on the side of caution, I did drill a few holes in the freeze plug.  I figured this was a good safety measure to keep flow in the case that I didn't go far enough in.  Figured I could always add another plug immediately after the first one if needed, if the holes allowed too much flow.

Fired it up and is much quieter and sounds more like it used to.  Still having pretty major backfiring though (needle jet shimmed with 2 washers and idle screw adjusted already)

Verslagen,
Will try this.  This is the first start up of the 'new' engine you may have read about in my gaskets for sale thread.  So it is possible there is a leak around the intake or something like that.  
The bike sat for some months after the old engine died but I don't remember it backfiring this[i] much.
The swap and reasons are a different story, maybe I will write up a separate post.[/i]

To try and stay on topic here, advice for anyone hesitant is that the plug was really cheap and easy to hammer in.  I used a junk socket and extension to hammer it in.