Assuming your talking about the Low Rider, you can knock out the plug in the center of the mufflers. If you search the Forum there are several threads on how to do it. I have a set of stock mufflers from an 02 that have the plug opened and also a set of the SEII. Either set is for sale cheap but it may cost more than they are worth to ship to Japan. PM me if you are interested.
The Dyna stock slip-on will not sound good compared to after market slip-ons. No driling/punching gutting or what ever will do it. Compleate gutting take hours and the loss of back psi make them do little more than make noise.
Knock out the pulgs is the best way to go. You keep some back PSI and they sound good enought till the $70 dyno power slip-on get ordered. Its easy too.
After I punched the plugs out on my 2002 Dyna (I like the sound too...) the bike ran just fine except for some exhaust popping on decel. A month or 2 later I rejetted and put a Ness Big Sucker on. Now the exhaust popping is 95% gone and power is right on...