There are 2 slits at front of muffler.Create 2 more slits with a hacksaw (so that you have a total of 4 slits. ( at 90,180,270,360 degrees).
You shouls have 4 tabs on the Mac where it slips onto the headerpipe.Bend these tabs out very slightly (if need too) for ease of slipping it on to the header. Put clamp on Mac (very loose). Now take the factory gasket and slip it on the HEADERPIPE  till the STOP (Big ridge). Slip on MAC over gasket till the same STOP. DO NOT SLIDE MAC OVER THIS STOP! You should be able to see the gasket trough the slits. If the gasket doesn't cover the slits all the way use a LITTLE high temp sealer. Slide clamp forward over slits and tighten. The clamp should be clamped over the slits with the gasket underneath, since this gasket is also your  SPACER.  Make sure the most rear muffler bolt (where it bolts onto the carrier)has the supplied spacer to raise the mac up a bit. Heatshields are Harley's.