I don't think that Flowmasters were O.E. The pipes from your system may be stock though. I've heard alot about getting 'drone' from removing the resonators on a B-body, especially if you don't have an H-pipe or a crossover... If you want specific info about your car you may want to check out this forum too....
http://impalassforum.com/cgi-bin/ultimatebb.cgi
They are a great bunch of guys that love everything and anything to do with B-bodies. Lots of info that's vehicle specific, including tried and tested mods.
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1977 GMC Jimmy - daily driver-sitting on 33's and bedliner inside and out
1995 9C1 Caprice - winter beater
1994 Impala SS - newest project
1991 9C1 Caprice - (rip)
"There are two great problems in the world today; an ever-growing lack of common sense, and too many warning labels. Take away all of the warning labels and the problem will work itself out..." -B