I've noticed some people go with an adapter and bolt it up that way.  Some bolt it up direct.  I didn't want mine sticking out to far, so I bolted direct.  If you put a bunch of hacksaw cuts on the front end of the muffler, you can squeeze it down onto the stock pipe over the stock exhaust gasket (if you didn't destroy it) and drill a hole in the stock muffler support bracket to match up with the muffler's bolt tab.  Or if you like the longer exhaust line, pay a couple bucks for an adapter/extension at your local muffler shop.

My solution to fit a HD-Dyna muffler. Took a 6"long pipe that fits the inner diam. of the muffler. Sawed the front end off at an angle  and welded that to the existing header. The muff was clamped with an HD clamp to the pipe and ... NO leaks.
The rear end of the muff. attached with a bolt to the existing bracket. As a matter of safety ( I'm from aviation maintenance) I drilled the head of the bolt and fitted locking wire.
You renew the header-to-head gasket offcourse!

Thanks for the tips guys. I don't think I will weld anything yet. I may want to do Jardine or Super Trapp in the future.  I think I can slip the HD over the stock header but the mount may be an issue. I like the idea of fabbing a bracket if the mounts do not match. Heh Heh, Metal Fab-GOOD!