I marked the distance to be cut all the way around using a ruler. Then put blue tape on the mark and cut very carefully with a hack say. Used a bench grinder to smooth out the edges. Cut the core to length, drilled new end cap holes and put it all together with new packing... Most reputable muffler shops ought to be able to do the same...
Good Luck,
PS: I took some off both ends to let me use what was otherwise a destroyed Mark's Can, turned out great..