I would go with an 18" Magnaflow as well... a more performance sound to it than Flows. Although if you're stuck on Flowmaster I would run a 50 series... they are the best sounding ones IMO.

 

I agree with the drone issues with flowmaster, just something thats common. If I were you I would go with borla or magnaflow because of the straight through design.

Here is a sound clip of a Borla exhaust (I have the same one on my truck)
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Also a link to where I bought my muffler:
http://www.summitracing.com/parts/BOR-40349/

 

I hear ya, my MAC's drone bad on my 5.0 also