[famoroso]

This is the rare instance where you want the bolts to snap (decisively and quickly). Start there. Then, if you can't get a box end wrench on what's left of the nuts & bolts... HEAT, (although Oxy isn't in the purview of the home "mechanic").

Best Case scenario is that all three bolts snap AND the strap bolts come off (otherwise, use a grinder to cut the straps off).

Then, grab the muffler tip and the right side of the muffler and crank it off as you watch the nut & bolt remnants, gasket and rust dust hit the floor.

May the odds be ever in your favor.