<VV> Muffler resonating

[James Cuneo]

Hello, I have a 180 turbo using a flowmaster muffler with 6" tail pipe.Around town and at low speed cruising the sound is great but, at rpms above 2500 there is  to much resonation in the car. It makes it uncomfortable to drive at highway speeds and listen to the stereo.I have used FatMat throughout the interior to dampen noise but this does not seem to help with the exhaust.Can someone recommend a method to dampen this muffler or recommend a muffler to help with this problem.The car was originally a 140 so does not have the cutout for the exhaust tip. I would use a stock muffler but do not want to cut the body. Anyone have experience with Magnaflow?Thanks,Jim Cuneo65 Corsa with turbo 140Sonoma County CA