Jim, leave the factory exhaust alone and the Cummins is as quiet as you will find. Much quieter than a Ferd. No experience with a Duramax so it might be an option. Diesels are noisy just no way around it. Sounds like a fun project that several on here have done. Do a search and send some PM's to the original posters for their experience. Good luck!
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Martin
'62 Chevy C-10 Stepside Shortbed (Restomod in progress)
'47 Chevy 3100 5 Window (long term project)
‘65 Chevy Biscayne (Emily)
‘39 Dodge Business Coupe (Clarence)
“I fought the law and the law won" now I are a retired one!
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USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)