Unless you bought the Mufflex kit which is a Magnaflow muffler with pipe extensions welded on, you WILL need some pipe to extend the muffler. Mufflers on their own are significantly shorter than the enormous stock muffler.
You may want to consider having a shop weld it on. If you use pipe there will be several joints which can not only leak, but be prone to vibration, and, well, falling apart. Two of us installed my 22" Magnaflow muffler with the widest band clamps we could find, and the passenger side exhaust after the muffler fell off a week afterwards, scraping up the side of the tip as it dragged along the road until I could pull over. Not funny.
We're both mechanically inclined, didn't take short cuts and it still didn't end up well. I got it welded after that, and hope this doesn't happen to anyone else.