To start off, it's not a huge question because I think I know the answer already, but with a single in/single out muffler on the back of the car, will there be any performance gain or loss based on pressure from the exhaust turning on the dual outlet or being strangled by the single? And does anyone happen to have that setup with something from Pypes? My dad's '67 used to run 2 Pypes Street Pros and I think it would sound great to use one of them but maybe be a little loud on the highway with my 358 at 10:1. Stepping up the compression definitely made the car louder, and as a daily driver I'm looking for something a bit more tame than whatever Flowmaster or Flowmaster knockoff is on it now. My other consideration is to put in a cutout with something normally quiet out back, and put a blackwidow exhaust Widowmaker 10" out the side of the vehicle for when I want something loud. I can throw in a little diagram of what I'm thinking of, as I'm not sure if it will clear the floor pan or not. My current setup is SLP shorty headers and Y-pipe going into a magnaflow hi flow cat (PA inspection law, it is my daily driver), and necking down from 3" to 2.5" after the cat to use the existing exhaust piping on a budget. I know I should be going up to a 3 inch if I'm looking at making ~370-400chp at the end of everything, I just haven't done it yet.

Dark blue for the cutout, light blue piping, and then red for the cylindrical muffler going out the side.

 

There are 3.0, 3.5, 4.0 inch exhaust systems available. Single in and single out muffler will give better results, especially if you can see straight through it. Just hide the single tailpipe behind the rear bumper with a turn down. Sewer pipe sticking out the back looks goofy.

 

I think a single 3" full exhaust will be sufficient. I had a dyno don's 3" single exhaust with a black widow neighborhater muffler and the car absolutely screamed. Was honestly too loud for city driving so I put the flowmaster 2-1 back on.

 

Comparing the same cross sections, everything else being equal the single will always flow more, with fewer losses to the walls of the tubing (the single will have less surface area).

Be careful with cutouts, they're becoming illegal in a lot of areas, they are in MD now.

I don't know how you intend to run the muffler after the cutout, but there isn't much room for one.

In general, chambered mufflers don't flow as well as straight-through designes, but that's not always the case, same for crossflow vs more traditional layouts.

 

I remember an old post here about someone posting better 1/4 mile times with 1 in / 1 out vs 1 in / 2 out....
I got a Flowmaster 1 in / 1 out and it also hides the tail pipe, discreetly, which I prefer to having 1 or 2 or any pipes shooting out the back (to eac, his own)... I even engine primered / engine painted it black
My neighbor had a Magnaflow muffler, 1 in 1 out on his 89 Camaro, I'm getting that one next time

Rafael