Flowmaster drone solved.

JohnK, just saw this buried in the string; sorry for being so late offering an answer. The X will help but it will not get rid of the flowmaster drone. if it were me, I'd adapt Dynomax 17748 muffler son and be done with most, if not all, drone and you will also enjoy enough exhaust flow to handle your power PLUS it still sounds like a 340 should. I ran mine for years but just put them on an LS3 powered 1957 Chevy Truck to get rid of noise from magna flow short bodied straight through mufflers. Hope this offers an alternative that helps you.
Tom

Voetom, I'd like your opinion. I have a 1970 Duster 340 with 833 trans. It currently has OEM manifold,
2 1/2" H-pipe exhaust with FlowMaster 40's and piped to the tips. I like the sound but do have drone at cruising speed. I am redoing exhaust using TTI long pipe headers, 2 1/2" X-pipe exhaust to the tips. Would like your opinion on mufflers. Engine Masters showed that X-pipe exhaust quiets the sound so still looking for that muscle car sound out of my system. Street car use mainly. Your thoughts? Thanks.