I use Teflon Plumbers tape on my YS 63 with stock muffler, and my YS 1.40 DZ with a Performance Specialties nipple and muffler. It works great, both have stayed on tight. I use about 3 - 4 wraps on the threads at both the exhaust outlet and the muffler joints.
For tightening I ground down an old 14MM (I think right size) wrench thin enough to tighten the lock nut to Muffler flange.