How do Flowmasters kill HP? I know in my own seat-of-the-pants tests, the Flows may not make more HP, but I know they produce more torque. This has been on stock and mildly modified 350's though. Is there something about high HP engines that the Flows don't work well with?
I personally don't really care for the sound of Magnaflows, but they have good reviews. A buddy of mine had a set put on a 650HP 502BB with 3" pipes and an X-pipe. Alothough it did sound decent, it certainly didn't sound like a 650HP BB should sound.
I know FLowmasters are louder than most mufflers and they defintely bring out the sound of the lopes of a good sounding cam. If you want to stay with Flows, I say go with a 50 series. They're not as loud as the 40 series. Also, run your pipes out to the rear of the truck. The shorter pipes and/or turndowns just make everything louder.