I've had flowmasters, dynomax, spintech, and Mac Flowpaths on my Mustangs. For what it's worth, I just removed a set of 40 series flows on my 90 and replaced them with SS Flowpath's. For a fox body I don't think there is a better sounding muffler.
Flows are great but I get sick of the drone they are known to throw off.
On a 3.8 flows sound great at idle, after that..... I would look for someone's stock cat back. It will bolt right up. Might not have that aggressive sound you are looking for but since you are missing 2 cylinders there really is no way to make it sound like you want.