My experience with Exhaust systems, if it helps. I ran 3” Dr Gas crossover. Exact same system as ( Pipes ) which was originally Dr Gas... with mufflers. 2” headers 3-1/2” collectors. So collector had to be reduce to 3”. Which I think is one of the killers...just guessing
Now I run 4” Merge collector, with 4” Bola stainless steel mufflers, with turn downs in front of rear axle. Car pick up .3 tenths over crossover system. Absolutely no difference with open headers in ET.
I pop off the turn downs and put cones in to meet the 90 decibel level for street
It as quiet as Pipes system.
Its A body car so plenty of room to run 4” pipe. Straight back from the headers. No fumes what so ever get in the car from the exhaust. Of cruse it only applies if you have the room.
Heres a video of what it sounds like with 4” and cones
https://youtu.be/I2gvpy0fNN8