EFI Tune for Quietest Possible Exhaust Noise

I dunno how you can incorporate any of the following, maybe it's just food for thought;
I have a carbureted, factory centrifugal-advance type Distributor, 11/1, 367 Mopar LA: a nice spicy 400hp engine.
I have a dash-mounted, adjustable Timing module, with a range of 15 degrees. I use it in several ways including;
> up to Plus 6* for cruise-timing
> Plus 2>4 Degrees for Power-Timing
> up to Minus 9* for a lower idle rpm, for parading, and
> the full Minus 9* for tip-toeing past cops. And, I have turn-downs on the pipe-ends .

Late ignition timing reduces my cylinder pressure out the tailpipes, which reduces the intensity of every pop, as the expanding gasses chase after the piston on the PowerStroke. But my Second cam had longer powerstroke, to suck the energy out of the expanding gasses and stuff it into the clutch. By the time she gets to the exhaust cycle, there's not a lot of energy left in the exhaust gasses.
The large, thick-walled, 3" exhaust pipes, and large 3-pass mufflers, suck the heat out of the exhaust, reducing it's volume and speed out the tail pipes. Plus then there's the turndowns on the end.
All in all, this system removes the hi-pitched, higher intensity, annoying sounds, leaving the lower bass tones .
You could maybe quiet it a lil more by fattening up the fuel curve as well.
At 3300rpm, you can take a lot of timing out.
Conversely, you could add timing which will increase the exhaust pressure, but the gasses will be cooler.
I think, a lil experimentation might be required.
Good luck