I had to use a die grinder with a cutoff wheel on mine. I just cut the pipe on one side and then opened up the outer pipe with an air chisel. I tried the comealong method, but think it would have worked better if i had two of them, one to hold the muffler back, as it will keep pulling forward in the bushings. I even used the torch to heat up the coupling, but it still wouldn't break apart for me without the cutoff wheel and chisel.
Anyway, once i had the pipe opened up a bit i could wiggle them apart. When i put it back i used a new clamp and since i didn't have to cut too much of the pipe, it sealed perfectly.
Also, I found the best method for breaking up the innards was a 5' chisel-ended bar. I just stood the cat on end on the floor of my shop and went to town. Took all of 3 minutes to clean it out.
Ken