Quiet mufflers

How to make a Mopar exhaust a sleeper?
I saw a 12 sec duster go down the track quiet as can be. Desperate to know, I asked the driver after the run what exhaust he had. He did not know b/c he bought the car from a friend. I looked under to see rusted pipes and unknown mufflers. (No power adders)

When I bought my duster it had Flowmasters= too loud
Changed to Magnaflows= no change
Now Dynomax= little change
(Mopar 380 HP crate motor, H pipe, tti headers)

I have cutouts, so maybe stock mufflers might be the last resort for the street. Over the winter, I pulled the interior and put in Eastwood's deadener on the firewall, floor and speaker tray. I hope it makes the drive more comfortable without the ridiculous drone and deafening noise where you can't have a conversation.