I'd go with a 40, even on a V6. I haven't heard a cat-back system yet that I thought was too loud, I guess because I am used to the true duals and glasspacks on my Mirada and Charger.
My father has a Flo-Pro single in, dual out muffler on his V6 Dakota and it sounds mellow, not loud, even with no cat. I know its not a Flowmaster, but I believe it's similar.
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