On my 04 I have a 50 series Delta Flow Flowmaster....the inet and outlet tube are both 2.5". You will need about a 6-8" extension of pipe (don't remember exactly) I had mine installed @ Midas where they just welded the extension to the front of the muffler and welded the muffler at the rear of the connecting flange so that you can still remove the whole rear of the exhaust just like from factory and the rear was welded at the bottom of the up turn before the axle. no need to take into account a smaller dia. Oh, and it sounds great...they said I was the first liberty they put a performance muffler on and they loved the sound.
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'04 Bright Silver Liberty 2wd
http://www.cardomain.com/ride/820370/1
L.O.S.T. Lifetime member JC050396
Va. Beach, Va.