You don't pull the muffler back away from the cat, if you're replacing the cat you pull the cat out of the muffler.
What you will probably have to do is slit the inlet sleeve of the muffler and stick a flat screwdriver in there to open it up a bit. They make a special tool for this purpose, but if you don't have one it may not be worth buying for the one job. Make a little space for the pipe to wiggle, then you can twist it around and pull it out.
You certainly should not need a come-along. If you have to pull that hard, you haven't loosened things up sufficiently.