I have a 2006 RK with 28k on the Samson headers and Rush mufflers using the OEM clamps with no leaking or slipping problem.

 

This works and old school is to use a beer can for the sheet metal source.

 

The bolts and H-spacer on the older OEM clamps have a spline which has to be aligned or it tries to cut a new spline while tightening, and this can keep them from torquing correctly. Also, look at the muffler ends. My old style SE Slash Cuts have slits cut to allow for compression when tightening the clamps. Mine could have compressed the header pipe, but those slits were not wide enough to allow the slip-ons to clamp onto the header pipe. I widened the slits with a cutting wheel...

 

whenever i install mufflers, i'll torque the clamp bolts properly, go for a ride, come back home and re torque them. a little black hi temp permatex silicone sealer will help also. you can easily wipe off any that squirts out of the joint and it looks clean, seals well too.

 

You may also have put the clamps too far back towards the muffler instead of where the split is.