I have always been a fan of flowmasters.  I ran a 40 series on my old 2.5L and it was too loud for my taste, and I also like it loud ( I run Vance and Hines on my Road King).  I run a 50 series with the 4.0L and I like the sound a lot better.  Nice throaty sound, at idle and when you give it gas, but not so loud on the highway that you can't hear your self think.  A few years back it cost me about $150 for the muffler and a custom bent 2 1/2" tail pipe, installed.  Just my .02 cents.