So here we go . . I had to mod the trunk in order to get those M3 muffler hangers installed. I had to grind off the old studs with a bandfile, and I replaced them with 5/16-18 x 1.5" bolts (the 4 white arrows). The blue arrows are what the old studs looked like:
And here's one of the new hangers installed, you can see the new bolt:
Just thought I'd share this for anyone out there who's thinking of using the M3 hangers.