There's a big difference between running turndowns in front of the rear axle and running pipes out to the bumper. But if your tailpipes run to the back bumper and end in turndowns, changing the shape of the last 3" isn't going to change a thing.

One thing to think about is the floor is like a big speaker. You can reduce sound levels a lot by damping this area. There are many choices such as Dynamat, Fatmat, Lizardskin, and asphalt roofing material. I don't fancy my car smelling like the La Brea tar pits on a hot day, so the roofing material is out for me.

The trunk floor is another prime candidate. Then there is the hood and firewall. Lizardskin is applied by spraying so for the trunk and under hood areas it might be best.
You may not want to be spraying in your car interior so one of the peel and stick products would be more readably utilized. Reading the websites I see there is more to the stuff than simple mass loading, which is what the asphalt roofing material does.

Here's an experiment for you: there are a lot of flooring products that come in one foot squares and are peel and stick. You cold run an experiment for just a few dollars. Apply the flooring material to your interior floor and trunk floor. See if it makes a difference. It won't cost a lot as I've bought squares at one of the dollar stores for a dollar a square foot.

I didn't read all the posts so I cant remember if you have an H or X pipe already. If you don't have one, get one in there.

The bullet resonators will also have a great effect on sound. Back in the day there was something called "chambered exhaust" Where the exhaust pipe was deformed so it acted like a muffler. I wonder if anyone has aomething like that to act as a resonator and not take up so much space.

Oh, and nice Dart! It looks super smooth and clean.

Good Luck,
R.

Then there's sound cancelling technology.........