(copied from my post in the above link "installing Fullsac baffles)
One thing I found out by experimenting with the different Dremel grinding wheels and rotary files: if used very carefully and lightly, the tiny cutoff abrasive wheels will do the trick in 5-10 minutes whilst using the edge of them like a grinder. I had to cut off the 'stingers' on the old stock baffles(can you say sawzall?) to get clearance at angles to allow the cutoff wheel to move around enough. Just be very careful not to undercut(cut too deep) the muffler body, as these wheels cut very quickly.
Another thing I did was to drill two holes in the top of my work bench that have the same spacing as the muffler mounts. Then run bolts from underneath to hold the mufflers while I worked. (towel, pad, or rag under front of muffs to prevent scratches). I suppose I could've built a jig like this to go in a vise, also. I found this very secure to prevent damage to the muff bodies and provide a stable working mount.
Be sure and wear your safety glasses and remember you've got sparks and tender.
Good luck....YMMV
Hope this helps.....